Patent Publication Number: US-2023146346-A1

Title: Recombinant fusion proteins for producing milk proteins in plants

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 17/851,624, filed Jun. 28, 2022, which is a continuation of U.S. application Ser. No. 17/374,125, filed Jul. 13, 2021, now issued as U.S. Pat. No. 11,401,526 on Aug. 2, 2022, which is a division of U.S. application Ser. No. 17/157,105, filed Jan. 25, 2021 now issued as U.S. Pat. No. 11,072,797 on Jul. 27, 2021, which is a continuation of U.S. application Ser. No. 17/039,759, filed Sep. 30, 2020 now issued as U.S. Pat. No. 10,947,552 on Mar. 16, 2021, the disclosures of each of which are hereby incorporated by reference in their entireties. 
    
    
     REFERENCE TO AN ELECTRONIC SEQUENCE LISTING 
     The contents of the electronic sequence listing (ALRO_007_17US_SeqList_ST26.xml; Size: 183,743 bytes; and Date of Creation: Jan. 20, 2023) are herein incorporated by reference in its entirety. 
     FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to recombinant milk proteins, and methods of production, extraction, and purification of the milk proteins from transgenic plants. The disclosure also relates to food compositions comprising recombinant milk proteins. 
     BACKGROUND 
     Globally, more than 7.5 billion people around the world consume milk and milk products. Demand for cow milk and dairy products is expected to keep increasing due to increased reliance on these products in developing countries as well as growth in the human population, which is expected to exceed 9 billion people by 2050. 
     Relying on animal agriculture to meet the growing demand for food is not a sustainable solution. According to the Food &amp; Agriculture Organization of the United Nations, animal agriculture is responsible for 18% of all greenhouse gases, more than the entire transportation sector combined. Dairy cows alone account for 3% of this total. 
     In addition to impacting the environment, animal agriculture poses a serious risk to human health. A startling 80% of antibiotics used in the United States go towards treating animals, resulting in the development of antibiotic resistant microorganisms also known as superbugs. For years, food companies and farmers have administered antibiotics not only to sick animals, but also to healthy animals, to prevent illness. In September 2016, the United Nations announced the use of antibiotics in the food system as a crisis on par with Ebola and HIV. 
     It is estimated that cow milk accounts for 83% of global milk production. Accordingly, there is an urgent need for to provide bovine milk and/or essential high-quality proteins from bovine milk in a more sustainable and humane manner, instead of solely relying on animal farming. Also, there is a need for selectively producing the specific milk proteins that confer nutritional and clinical benefits, and/or do not provoke allergic responses. 
     BRIEF SUMMARY 
     Provided herein are compositions and methods for producing milk proteins in transgenic plants. In some embodiments, a milk protein is stably expressed in a transgenic plant by fusing it to a stable protein, such as a stable mammalian, avian, plant or fungal protein. The compositions and methods provided herein allow for safe, sustainable and humane production of milk proteins for commercial use, such as use in food compositions. 
     In some embodiments, the disclosure provides a stably transformed plant comprising in its genome: a recombinant DNA construct encoding a fusion protein, the fusion protein comprising: (i) an unstructured milk protein, and (ii) a structured animal protein; wherein the fusion protein is stably expressed in the plant in an amount of 1% or higher per total protein weight of soluble protein extractable from the plant. 
     In some embodiments, the disclosure provides a stably transformed plant, comprising in its genome: a recombinant DNA construct encoding a fusion protein, the fusion protein comprising: κ-casein; and β-lactoglobulin; wherein the fusion protein is stably expressed in the plant in an amount of 1% or higher per total protein weight of soluble protein extractable from the plant. 
     In some embodiments, the disclosure provides a recombinant fusion protein comprising: (i) an unstructured milk protein, and (ii) a structured animal protein. 
     In some embodiments, the disclosure provides a plant-expressed recombinant fusion protein, comprising: κ-casein and β-lactoglobulin. 
     Also provided are nucleic acids encoding the recombinant fusion proteins described herein. 
     Also provided are vectors comprising a nucleic acid encoding one or more recombinant fusion proteins described herein, wherein the recombinant fusion protein comprises: (i) an unstructured milk protein, and (ii) a structured animal protein. 
     Also provided are plants comprising the recombinant fusion proteins and/or the nucleic acids described herein. 
     The instant disclosure also provides a method for stably expressing a recombinant fusion protein in a plant, the method comprising: a) transforming a plant with a plant transformation vector comprising an expression cassette comprising: a sequence encoding a fusion protein, wherein the fusion protein comprises an unstructured milk protein, and a structured animal protein; and b) growing the transformed plant under conditions wherein the recombinant fusion protein is expressed in an amount of 1% or higher per total protein weight of soluble protein extractable from the plant. 
     Also provided herein are methods for making food compositions, the methods comprising: expressing the recombinant fusion protein in a plant; extracting the recombinant fusion protein from the plant; optionally, separating the milk protein from the structured animal protein or the structured plant protein; and creating a food composition using the milk protein or the fusion protein. 
     Also provided herein are food compositions comprising one or more recombinant fusion proteins as described herein. 
     Also provided are food compositions produced using any one of the methods disclosed herein. 
     These and other embodiments are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, which are incorporated herein and form a part of the specification, illustrate some, but not the only or exclusive, example embodiments and/or features. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than limiting. 
         FIGS.  1 A,  1 B,  1 C,  1 D,  1 E,  1 F,  1 G,  1 H,  1 I,  1 J,  1 K,  1 L,  1 M , I 1 ,  1 O, and  1 P show expression cassettes having different combinations of fusions between structured and intrinsically unstructured proteins (not to scale). Coding regions and regulatory sequences are indicated as blocks (not to scale). As used in the figures, “L” refers to linker; “Sig” refers to a signal sequence that directs foreign proteins to protein storage vacuoles, “5′ UTR” refers to the 5′ untranslated region, and “KDEL” refers to an endoplasmic reticulum retention signal. 
         FIG.  2    shows the modified pAR15-00 cloning vector containing a selectable marker cassette conferring herbicide resistance. Coding regions and regulatory sequences are indicated as blocks (not to scale). 
         FIG.  3    shows an example expression cassette comprising a OKC1-T:OLG1 (Optimized Kappa Casein version 1:beta-lactoglobulin version 1, SEQ ID NOs: 71-72) fusion driven by PvPhas promoter fused with arc5′UTR:sig10, followed by the ER retention signal (KDEL) and the 3′UTR of the arc5-1 gene, “arc-terminator”. “arc5′UTR” refers to the 5′ untranslated region of the arc5-1 gene. “Sig10” refers to the lectin 1 gene signal peptide. “RB” refers to ribosomal binding site. Coding regions and regulatory sequences are indicated as blocks (not to scale). 
         FIG.  4    shows an example expression cassette comprising a OBC-T2:FM:OLG1 (Optimized Beta Casein Truncated version 2:Chymosin cleavage site:beta-lactoglobulin version 1, SEQ ID NOs: 73-74) fusion driven by PvPhas promoter fused with arc5′UTR:sig10, followed by the 3′UTR of the arc5-1 gene, “arc-terminator”. “arc5′UTR” refers to the 5′ untranslated region of the arc5-1 gene. “Sig10” refers to the lectin 1 gene signal peptide. “RB” refers to ribosomal binding site. Coding regions and regulatory sequences are indicated as blocks (not to scale). The Beta Casein is “truncated” in that the bovine secretion signal is removed, and replaced with a plant targeting signal. 
         FIG.  5    shows an example expression cassette comprising a OaS1-T:FM:OLG1 (Optimized Alpha S1 Casein Truncated version 1:Chymosin cleavage site:beta-lactoglobulin version 1, SEQ ID NOs: 75-76) fusion driven by PvPhas promoter fused with arc5′UTR:sig10, followed by the 3′UTR of the arc5-1 gene, “arc-terminator”. “arc5′UTR” refers to the 5′ untranslated region of the arc5-1 gene. “Sig10” refers to the lectin 1 gene signal peptide. “RB” refers to ribosomal binding site. Coding regions and regulatory sequences are indicated as blocks (not to scale). The Alpha S1 Casein is “truncated” in that the bovine secretion signal is removed, and replaced with a plant targeting signal. 
         FIG.  6    shows an example expression cassette comprising a para-OKC1-T:FM:OLG1:KDEL (Optimized paraKappa Casein version 1:Chymosin cleavage site:beta-lactoglobulin version 1, SEQ ID NOs: 77-78) fusion driven by PvPhas promoter fused with arc5′UTR:sig 10, followed by the ER retention signal (KDEL) and the 3′UTR of the arc5-1 gene, “arc-terminator”. “arc5′UTR” refers to the 5′ untranslated region of the arc5-1 gene. “Sig10” refers to the lectin 1 gene signal peptide. “RB” refers to ribosomal binding site. Coding regions and regulatory sequences are indicated as blocks (not to scale). 
         FIG.  7    shows an example expression cassette comprising a para-OKC1-T:FM:OLG1 (Optimized paraKappa Casein version 1:Chymosin cleavage site:beta-lactoglobulin version 1, SEQ ID NOs: 79-80) fusion driven by PvPhas promoter fused with arc5′UTR:sig 10, followed by the 3′UTR of the arc5-1 gene, “arc-terminator.” “arc5′UTR” refers to the 5′ untranslated region of the arc5-1 gene. “Sig10” refers to the lectin 1 gene signal peptide. “RB” refers to ribosomal binding site. Coding regions and regulatory sequences are indicated as blocks (not to scale). 
         FIG.  8    shows an example expression cassette comprising a OKC1-T:OLG1 (Optimized Kappa Casein version 1:beta-lactoglobulin version 1, SEQ ID NOs: 81-82) fusion that is driven by the promoter and signal peptide of glycinin 1 (GmSeed2:sig2) followed by the ER retention signal (KDEL) and the nopaline synthase gene termination sequence, (nos term). Coding regions and regulatory sequences are indicated as blocks (not to scale). 
         FIGS.  9 A,  9 B,  9 C, and  9 D  show protein detection by western blotting.  FIG.  9 A  shows detection of the fusion protein using a primary antibody raised against κ-casein (kCN). The kCN commercial protein is detected at an apparent MW of ˜26 kDa (theoretical: 19 kDa-arrow). The fusion protein is detected at an apparent MW of ˜40 kDa (theoretical: 38 kDa—arrowhead).  FIG.  9 B  shows detection of the fusion protein using a primary antibody raised against β-lactoglobulin (LG). The LG commercial protein is detected at an apparent MW of ˜18 kDa (theoretical: 18 kDa-arrow). The fusion protein is detected at an apparent MW of ˜40 kDa (theoretical: 38 kDa-arrowhead).  FIGS.  9 C and  9 D  show protein gels as control for equal lane loading (image is taken at the end of the SDS run). 
         FIGS.  10 A and  10 B  show two illustrative fusion proteins. In  FIG.  10 A , a κ-casein protein is fused to a β-lactoglobulin protein. The κ-casein comprises a natural chymosin cleavage site (arrow 1). Cleavage of the fusion protein with rennet (or chymosin) yields two fragments: a para-kappa casein fragment, and a fragment comprising a κ-casein macropeptide fused to β-lactoglobulin. In some embodiments, a second protease cleavage site may be added at the C-terminus of the k-casein protein (i.e., at arrow 2), in order to further allow separation of the κ-casein macropeptide and the β-lactoglobulin. The second protease cleavage site may be a rennet cleavage site (e.g., a chymosin cleavage site), or it may be a cleavage site for a different protease. In  FIG.  10 B , a para-κ-casein protein is fused directly to β-lactoglobulin. A protease cleavage site (e.g., a rennet cleavage site) is added between the para-κ-casein and the β-lactoglobulin to allow for separation thereof. By fusing the para-κ-casein directly to the β-lactoglobulin, no κ-casein macropeptide is produced. 
         FIG.  11    is a flow-chart showing an illustrative process for producing a food composition comprising an unstructured milk protein, as described herein. 
     
    
    
     DETAILED DESCRIPTION 
     The following description includes information that may be useful in understanding the present disclosure. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed disclosures, or that any publication specifically or implicitly referenced is prior art. 
     Definitions 
     While the following terms are believed to be well understood by one of ordinary skill in the art, the following definitions are set forth to facilitate explanation of the presently disclosed subject matter. 
     All technical and scientific terms used herein, unless otherwise defined below, are intended to have the same meaning as commonly understood by one of ordinary skill in the art. References to techniques employed herein are intended to refer to the techniques as commonly understood in the art, including variations on those techniques and/or substitutions of equivalent techniques that would be apparent to one of skill in the art. 
     As used herein, the singular forms “a,” “an,” and “the: include plural referents unless the content clearly dictates otherwise. 
     The term “about” or “approximately” when immediately preceding a numerical value means a range (e.g., plus or minus 10% of that value). For example, “about 50” can mean 45 to 55, “about 25,000” can mean 22,500 to 27,500, etc., unless the context of the disclosure indicates otherwise, or is inconsistent with such an interpretation. For example, in a list of numerical values such as “about 49, about 50, about 55, . . . ”, “about 50” means a range extending to less than half the interval(s) between the preceding and subsequent values, e.g., more than 49.5 to less than 52.5. Furthermore, the phrases “less than about” a value or “greater than about” a value should be understood in view of the definition of the term “about” provided herein. Similarly, the term “about” when preceding a series of numerical values or a range of values (e.g., “about 10, 20, 30” or “about 10-30”) refers, respectively to all values in the series, or the endpoints of the range. 
     As used herein, “mammalian milk” can refer to milk derived from any mammal, such as bovine, human, goat, sheep, camel, buffalo, water buffalo, dromedary, llama and any combination thereof. In some embodiments, a mammalian milk is a bovine milk. 
     As used herein, “structured” refers to those proteins having a well-defined secondary and tertiary structure, and “unstructured” refers to proteins that do not have well defined secondary and/or tertiary structures. An unstructured protein may also be described as lacking a fixed or ordered three-dimensional structure. “Disordered” and “intrinsically disordered” are synonymous with unstructured. 
     As used herein, “rennet” refers to a set of enzymes typically produced in the stomachs of ruminant mammals. Chymosin, its key component, is a protease enzyme that cleaves κ-casein (to produce para-κ-casein). In addition to chymosin, rennet contains other enzymes, such as pepsin and lipase. Rennet is used to separate milk into solid curds (for cheesemaking) and liquid whey. Rennet or rennet substitutes are used in the production of most cheeses. 
     As used herein “whey” refers to the liquid remaining after milk has been curdled and strained, for example during cheesemaking. Whey comprises a collection of globular proteins, typically a mixture of β-lactoglobulin, α-lactalbumin, bovine serum albumin, and immunoglobulins. 
     The term “plant” includes reference to whole plants, plant organs, plant tissues, and plant cells and progeny of same, but is not limited to angiosperms and gymnosperms such as Arabidopsis, potato, tomato, tobacco, alfalfa, lettuce, carrot, strawberry, sugarbeet, cassava, sweet potato, soybean, lima bean, pea, chick pea, maize (corn), turf grass, wheat, rice, barley, sorghum, oat, oak, eucalyptus, walnut, palm and duckweed as well as fern and moss. Thus, a plant may be a monocot, a dicot, a vascular plant reproduced from spores such as fern or a nonvascular plant such as moss, liverwort, hornwort and algae. The word “plant,” as used herein, also encompasses plant cells, seeds, plant progeny, propagule whether generated sexually or asexually, and descendants of any of these, such as cuttings or seed. Plant cells include suspension cultures, callus, embryos, meristematic regions, callus tissue, leaves, roots, shoots, gametophytes, sporophytes, pollen, seeds and microspores. Plants may be at various stages of maturity and may be grown in liquid or solid culture, or in soil or suitable media in pots, greenhouses or fields. Expression of an introduced leader, trailer or gene sequences in plants may be transient or permanent. 
     The term “vascular plant” refers to a large group of plants that are defined as those land plants that have lignified tissues (the xylem) for conducting water and minerals throughout the plant and a specialized non-lignified tissue (the phloem) to conduct products of photosynthesis. Vascular plants include the clubmosses, horsetails, ferns, gymnosperms (including conifers) and angiosperms (flowering plants). Scientific names for the group include Tracheophyta and Tracheobionta. Vascular plants are distinguished by two primary characteristics. First, vascular plants have vascular tissues which distribute resources through the plant. This feature allows vascular plants to evolve to a larger size than non-vascular plants, which lack these specialized conducting tissues and are therefore restricted to relatively small sizes. Second, in vascular plants, the principal generation phase is the sporophyte, which is usually diploid with two sets of chromosomes per cell. Only the germ cells and gametophytes are haploid. By contrast, the principal generation phase in non-vascular plants is the gametophyte, which is haploid with one set of chromosomes per cell. In these plants, only the spore stalk and capsule are diploid. 
     The term “non-vascular plant” refers to a plant without a vascular system consisting of xylem and phloem. Many non-vascular plants have simpler tissues that are specialized for internal transport of water. For example, mosses and leafy liverworts have structures that look like leaves, but are not true leaves because they are single sheets of cells with no stomata, no internal air spaces and have no xylem or phloem. Non-vascular plants include two distantly related groups. The first group are the bryophytes, which is further categorized as three separate land plant Divisions, namely Bryophyta (mosses), Marchantiophyta (liverworts), and Anthocerotophyta (hornworts). In all bryophytes, the primary plants are the haploid gametophytes, with the only diploid portion being the attached sporophyte, consisting of a stalk and sporangium. Because these plants lack lignified water-conducting tissues, they can&#39;t become as tall as most vascular plants. The second group is the algae, especially the green algae, which consists of several unrelated groups. Only those groups of algae included in the Viridiplantae are still considered relatives of land plants. 
     The term “plant part” refers to any part of a plant including but not limited to the embryo, shoot, root, stem, seed, stipule, leaf, petal, flower bud, flower, ovule, bract, trichome, branch, petiole, internode, bark, pubescence, tiller, rhizome, frond, blade, ovule, pollen, stamen, and the like. The two main parts of plants grown in some sort of media, such as soil or vermiculite, are often referred to as the “above-ground” part, also often referred to as the “shoots”, and the “below-ground” part, also often referred to as the “roots”. 
     The term “plant tissue” refers to any part of a plant, such as a plant organ. Examples of plant organs include, but are not limited to the leaf, stem, root, tuber, seed, branch, pubescence, nodule, leaf axil, flower, pollen, stamen, pistil, petal, peduncle, stalk, stigma, style, bract, fruit, trunk, carpel, sepal, anther, ovule, pedicel, needle, cone, rhizome, stolon, shoot, pericarp, endosperm, placenta, berry, stamen, and leaf sheath. 
     The term “seed” is meant to encompass the whole seed and/or all seed components, including, for example, the coleoptile and leaves, radicle and coleorhiza, scutellum, starchy endosperm, aleurone layer, pericarp and/or testa, either during seed maturation and seed germination. 
     The term “transgenic plant” means a plant that has been transformed with one or more exogenous nucleic acids. “Transformation” refers to a process by which a nucleic acid is stably integrated into the genome of a plant cell. “Stably integrated” refers to the permanent, or non-transient retention and/or expression of a polynucleotide in and by a cell genome. Thus, a stably integrated polynucleotide is one that is a fixture within a transformed cell genome and can be replicated and propagated through successive progeny of the cell or resultant transformed plant. Transformation may occur under natural or artificial conditions using various methods well known in the art. Transformation may rely on any known method for the insertion of nucleic acid sequences into a prokaryotic or eukaryotic host cell, including Agrobacterium-mediated transformation protocols, viral infection, whiskers, electroporation, heat shock, lipofection, polyethylene glycol treatment, micro-injection, and particle bombardment. 
     As used herein, the terms “stably expressed” or “stable expression” refer to expression and accumulation of a protein in a plant cell over time. In some embodiments, a protein may accumulate because it is not degraded by endogenous plant proteases. In some embodiments, a protein is considered to be stably expressed in a plant if it is present in the plant in an amount of 1% or higher per total protein weight of soluble protein extractable from the plant. 
     As used herein, the term “fusion protein” refers to a protein comprising at least two constituent proteins (or fragments or variants thereof) that are encoded by separate genes, and that have been joined so that they are transcribed and translated as a single polypeptide. In some embodiments, a fusion protein may be separated into its constituent proteins, for example by cleavage with a protease. 
     The term “recombinant” refers to nucleic acids or proteins formed by laboratory methods of genetic recombination (e.g., molecular cloning) to bring together genetic material from multiple sources, creating sequences that would not otherwise be found in the genome. A recombinant fusion protein is a protein created by combining sequences encoding two or more constituent proteins, such that they are expressed as a single polypeptide. Recombinant fusion proteins may be expressed in vivo in various types of host cells, including plant cells, bacterial cells, fungal cells, mammalian cells, etc. Recombinant fusion proteins may also be generated in vitro. 
     The term “promoter” or a “transcription regulatory region” refers to nucleic acid sequences that influence and/or promote initiation of transcription. Promoters are typically considered to include regulatory regions, such as enhancer or inducer elements. The promoter will generally be appropriate to the host cell in which the target gene is being expressed. The promoter, together with other transcriptional and translational regulatory nucleic acid sequences (also termed “control sequences”), is necessary to express any given gene. In general, the transcriptional and translational regulatory sequences include, but are not limited to, promoter sequences, ribosomal binding sites, transcriptional start and stop sequences, translational start and stop sequences, and enhancer or activator sequences. 
     The term signal peptide—also known as “signal sequence”, “targeting signal”, “localization signal”, “localization sequence”, “transit peptide”, “leader sequence”, or “leader peptide”, is used herein to refer to an N-terminal peptide which directs a newly synthesized protein to a specific cellular location or pathway. Signal peptides are often cleaved from a protein during translation or transport, and are therefore not typically present in a mature protein. 
     The term “proteolysis” or “proteolytic” or “proteolyze” means the breakdown of proteins into smaller polypeptides or amino acids. Uncatalyzed hydrolysis of peptide bonds is extremely slow. Proteolysis is typically catalyzed by cellular enzymes called proteases, but may also occur by intra-molecular digestion. Low pH or high temperatures can also cause proteolysis non-enzymatically. Limited proteolysis of a polypeptide during or after translation in protein synthesis often occurs for many proteins. This may involve removal of the N-terminal methionine, signal peptide, and/or the conversion of an inactive or non-functional protein to an active one. 
     The term “2A peptide”, used herein, refers to nucleic acid sequence encoding a 2A peptide or the 2A peptide itself. The average length of 2A peptides is 18-22 amino acids. The designation “2A” refers to a specific region of picornavirus polyproteins and arose from a systematic nomenclature adopted by researchers. In foot-and-mouth disease virus (FMDV), a member of Picornaviridae family, a 2A sequence appears to have the unique capability to mediate cleavage at its own C-terminus by an apparently enzyme-independent, novel type of reaction. This sequence can also mediate cleavage in a heterologous protein context in a range of eukaryotic expression systems. The 2A sequence is inserted between two genes of interest, maintaining a single open reading frame. Efficient cleavage of the polyprotein can lead to co-ordinate expression of active two proteins of interest. Self-processing polyproteins using the FMDV 2A sequence could therefore provide a system for ensuring coordinated, stable expression of multiple introduced proteins in cells including plant cells. 
     The term “purifying” is used interchangeably with the term “isolating” and generally refers to the separation of a particular component from other components of the environment in which it was found or produced. For example, purifying a recombinant protein from plant cells in which it was produced typically means subjecting transgenic protein containing plant material to biochemical purification and/or column chromatography. 
     When referring to expression of a protein in a specific amount per the total protein weight of the soluble protein extractable from the plant (“TSP”), it is meant an amount of a protein of interest relative to the total amount of protein that may reasonably be extracted from a plant using standard methods. Methods for extracting total protein from a plant are known in the art. For example, total protein may be extracted from seeds by bead beating seeds at about 15000 rpm for about 1 min. The resulting powder may then be resuspended in an appropriate buffer (e.g., 50 mM Carbonate-Bicarbonate pH 10.8, 1 mM DTT, 1× Protease Inhibitor Cocktail). After the resuspended powder is incubated at about 4° C. for about 15 minutes, the supernatant may be collected after centrifuging (e.g., at 4000 g, 20 min, 4° C.). Total protein may be measured using standard assays, such as a Bradford assay. The amount of protein of interest may be measured using methods known in the art, such as an ELISA or a Western Blot. 
     When referring to a nucleic acid sequence or protein sequence, the term “identity” is used to denote similarity between two sequences. Sequence similarity or identity may be determined using standard techniques known in the art, including, but not limited to, the local sequence identity algorithm of Smith &amp; Waterman, Adv. Appl. Math. 2, 482 (1981), by the sequence identity alignment algorithm of Needleman &amp; Wunsch, J Mol. Biol. 48,443 (1970), by the search for similarity method of Pearson &amp; Lipman, Proc. Natl. Acad. Sci. USA 85, 2444 (1988), by computerized implementations of these algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package, Genetics Computer Group, 575 Science Drive, Madison, Wis.), the Best Fit sequence program described by Devereux et al., Nucl. Acid Res. 12, 387-395 (1984), or by inspection. Another suitable algorithm is the BLAST algorithm, described in Altschul et al., J Mol. Biol. 215, 403-410, (1990) and Karlin et al., Proc. Natl. Acad. Sci. USA 90, 5873-5787 (1993). A particularly useful BLAST program is the WU-BLAST-2 program which was obtained from Altschul et al., Methods in Enzymology, 266, 460-480 (1996); blast.wustl/edu/blast/README. WU-BLAST-2 uses several search parameters, which are optionally set to the default values. The parameters are dynamic values and are established by the program itself depending upon the composition of the particular sequence and composition of the particular database against which the sequence of interest is being searched; however, the values may be adjusted to increase sensitivity. Further, an additional useful algorithm is gapped BLAST as reported by Altschul et al, (1997) Nucleic Acids Res. 25, 3389-3402. As used herein, the terms “dicot” or “dicotyledon” or “dicotyledonous” refer to a flowering plant whose embryos have two seed leaves or cotyledons. Examples of dicots include, but are not limited to, Arabidopsis, tobacco, tomato, potato, sweet potato, cassava, alfalfa, lima bean, pea, chick pea, soybean, carrot, strawberry, lettuce, oak, maple, walnut, rose, mint, squash, daisy, Quinoa, buckwheat, mung bean, cow pea, lentil, lupin, peanut, fava bean, French beans (i.e., common beans), mustard, or cactus. 
     The terms “monocot” or “monocotyledon” or “monocotyledonous” refer to a flowering plant whose embryos have one cotyledon or seed leaf. Examples of monocots include, but are not limited to turf grass, maize (corn), rice, oat, wheat, barley, sorghum, orchid, iris, lily, onion, palm, and duckweed. 
     As used herein, a “low lactose product” is any food composition considered by the FDA to be “lactose reduced”, “low lactose”, or “lactose free”. 
     Unstructured Milk Proteins 
     The fusion proteins described herein may comprise one or more unstructured milk proteins. As used herein the term “milk protein” refers to any protein, or fragment or variant thereof, that is typically found in one or more mammalian milks. Examples of mammalian milk include, but are not limited to, milk produced by a cow, human, goat, sheep, camel, horse, donkey, dog, cat, elephant, monkey, mouse, rat, hamster, guinea pig, whale, dolphin, seal, sheep, buffalo, water buffalo, dromedary, llama, yak, zebu, reindeer, mole, otter, weasel, wolf, raccoon, walrus, polar bear, rabbit, or giraffe. 
     An “unstructured milk protein” is a milk protein that lacks a defined secondary structure, a defined tertiary structure, or a defined secondary and tertiary structure. Whether a milk protein is unstructured may be determined using a variety of biophysical and biochemical methods known in the art, such as small angle X-ray scattering, Raman optical activity, circular dichroism, nuclear magnetic resonance (NMR) and protease sensitivity. In some embodiments, a milk protein is considered to be unstructured if it is unable to be crystallized using standard techniques. 
     Illustrative unstructured milk proteins that may be used in the fusion proteins of the disclosure includes members of the casein family of proteins, such as α-S1 casein, α-S2 casein, β-casein, and κ-casein. The caseins are phosphoproteins, and make up approximately 80% of the protein content in bovine milk and about 20-45% of the protein in human milk. Caseins form a multi-molecular, granular structure called a casein micelle in which some enzymes, water, and salts, such as calcium and phosphorous, are present. The micellar structure of casein in milk is significant in terms of a mode of digestion of milk in the stomach and intestine and a basis for separating some proteins and other components from cow milk. In practice, casein proteins in bovine milk can be separated from whey proteins by acid precipitation of caseins, by breaking the micellar structure by partial hydrolysis of the protein molecules with proteolytic enzymes, or microfiltration to separate the smaller soluble whey proteins from the larger casein micelle. Caseins are relatively hydrophobic, making them poorly soluble in water. 
     In some embodiments, the casein proteins described herein (e.g., α-S1 casein, α-S2 casein, β-casein, and/or κ-casein) are isolated or derived from cow ( bos taurus ), goat ( capra hircus ), sheep ( ovis aries ), water buffalo ( bubalus bubalis ), dromedary camel ( camelus dromedaries ), bactrian camel ( camelus bactrianus ), wild yak ( bos mutus ), horse ( equus caballus ), donkey ( equus asinus ), reindeer ( rangifer tarandus ), eurasian elk ( alces alces ), alpaca ( vicugna pacos ), zebu ( bos indicus ), llama ( lama glama ), or human ( homo sapiens ). In some embodiments, a casein protein (e.g., α-S1 casein, α-S2 casein, β-casein, or κ-casein) has at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identity with a casein protein from one or more of cow ( bos taurus ), goat ( capra hircus ), sheep ( ovis aries ), water buffalo ( bubalus bubalis ), dromedary camel ( camelus dromedaries ), bactrian camel ( camelus bactrianus ), wild yak ( bos mutus ), horse ( equus caballus ), donkey ( equus asinus ), reindeer ( rangifer tarandus ), eurasian elk ( alces alces ), alpaca ( vicugna pacos ), zebu ( bos indicus ), llama ( lama glama ), or human ( homo sapiens ). 
     As used herein, the term “α-S1 casein” refers to not only the α-S1 casein protein, but also fragments or variants thereof. α-S1 casein is found in the milk of numerous different mammalian species, including cow, goat, and sheep. The sequence, structure and physical/chemical properties of α-S1 casein derived from various species is highly variable. An exemplary sequence for bovine α-S1 casein can be found at Uniprot Accession No. P02662, and an exemplary sequence for goat α-S1 casein can be found at GenBank Accession No. X59836.1. 
     As used herein, the term “α-S2 casein” refers to not only the α-S2 casein protein, but also fragments or variants thereof α-S2 is known as epsilon-casein in mouse, gamma-casein in rat, and casein-A in guinea pig. The sequence, structure and physical/chemical properties of α-S2 casein derived from various species is highly variable. An exemplary sequence for bovine α-S2 casein can be found at Uniprot Accession No. P02663, and an exemplary sequence for goat α-S2 casein can be found at Uniprot Accession No. P33049. 
     As used herein, the term “β-casein” refers to not only the β-casein protein, but also fragments or variants thereof. For example, A1 and A2 β-casein are genetic variants of the β-casein milk protein that differ by one amino acid (at amino acid 67, A2 β-casein has a proline, whereas A1 has a histidine). Other genetic variants of β-casein include the A3, B, C, D, E, F, H1, H2, I and G genetic variants. The sequence, structure and physical/chemical properties of β-casein derived from various species is highly variable. Exemplary sequences for bovine β-casein can be found at Uniprot Accession No. P02666 and GenBank Accession No. M15132.1. 
     As used herein, the term “κ-casein” refers to not only the κ-casein protein, but also fragments or variants thereof. κ-casein is cleaved by rennet, which releases a macropeptide from the C-terminal region. The remaining product with the N-terminus and two-thirds of the original peptide chain is referred to as para-κ-casein. The sequence, structure and physical/chemical properties of κ-casein derived from various species is highly variable. Exemplary sequences for bovine κ-casein can be found at Uniprot Accession No. P02668 and GenBank Accession No. CAA25231. 
     In some embodiments, the unstructured milk protein is a casein protein, for example, α-S1 casein, α-S2 casein, β-casein, and or κ-casein. In some embodiments, the unstructured milk protein is κ-casein and comprises the sequence of SEQ ID NO: 4, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the unstructured milk protein is para-κ-casein and comprises the sequence of SEQ ID NO: 2, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the unstructured milk protein is β-casein and comprises the sequence of SEQ ID NO: 6, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the unstructured milk protein is α-S1 casein and comprises the sequence SEQ ID NO: 8, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, unstructured milk protein is α-S2 casein and comprises the sequence SEQ ID NO: 84, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, the unstructured milk protein comprises a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to SEQ ID NO: 4. In some embodiments, the unstructured milk protein comprises a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to SEQ ID NO: 2. In some embodiments, the unstructured milk protein comprises a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to SEQ ID NO: 6. In some embodiments, the unstructured milk protein comprises a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to SEQ ID NO: 8. In some embodiments, the unstructured milk protein comprises a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to SEQ ID NO: 84. 
     In some embodiments, α-S1 casein is encoded by the sequence of SEQ ID NO: 7, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, α-S2 casein is encoded by the sequence of SEQ ID NO: 83, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, β-casein is encoded by the sequence of SEQ ID NO: 5, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, κ-casein is encoded by the sequence of SEQ ID NO: 3, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, para-κ-casein is encoded by the sequence of SEQ ID NO: 1, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, the unstructured milk protein is encoded by a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to SEQ ID NO: 7. In some embodiments, the unstructured milk protein is encoded by a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to SEQ ID NO: 83. In some embodiments, the unstructured milk protein is encoded by a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to SEQ ID NO: 3. In some embodiments, the unstructured milk protein is encoded by a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to SEQ ID NO: 1. In some embodiments, the unstructured milk protein is encoded by a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to SEQ ID NO: 5. 
     In some embodiments, the unstructured milk protein is a casein protein, and comprises a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to any one of SEQ ID NO: 85-133. In some embodiments, the unstructured milk protein is a casein protein and comprises the sequence of any one of SEQ ID NO: 85-133. 
     In some embodiments, the unstructured milk protein comprises a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to any one of SEQ ID NO: 85-98. In some embodiments, the unstructured milk protein comprises the sequence of any one of SEQ ID NO: 85-98. 
     In some embodiments, the unstructured milk protein comprises a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to any one of SEQ ID NO: 99-109. In some embodiments, the unstructured milk protein comprises the sequence of any one of SEQ ID NO: 99-109. 
     In some embodiments, the unstructured milk protein comprises a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to any one of SEQ ID NO: 110-120. In some embodiments, the unstructured milk protein comprises the sequence of any one of SEQ ID NO: 110-120. 
     In some embodiments, the unstructured milk protein comprises a sequence that is at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to any one of SEQ ID NO: 121-133. In some embodiments, the unstructured milk protein comprises the sequence of any one of SEQ ID NO: 121-133. 
     Structured Proteins 
     The fusion proteins described herein may comprise one or more structured proteins, including any fragment or variant thereof. The proteins may be, for example, structured animal proteins, or structured plant proteins. In some embodiments, the structured animal proteins are mammalian proteins. In some embodiments, the structured animal proteins are avian proteins. In some embodiments, the structured proteins are structured milk proteins. 
     Whether a milk protein is structured may be determined using a variety of biophysical and biochemical methods known in the art, such as small angle X-ray scattering, Raman optical activity, circular dichroism, and protease sensitivity. In some embodiments, a milk protein is considered to be structured if it has been crystallized or if it may be crystallized using standard techniques. 
     In some embodiments, the structured protein is not a protein that is typically used as a marker. As used herein, the term “marker” refers to a protein that produces a visual or other signal and is used to detect successful delivery of a vector (e.g., a DNA sequence) into a cell. Proteins typically used as a marker may include, for example, fluorescent proteins (e.g., green fluorescent protein (GFP)) and bacterial or other enzymes (e.g., β-glucuronidase (GUS), β-galactosidase, luciferase, chloramphenicol acetyltransferase). In some embodiments, the structured protein is a non-marker protein. 
     A non-limiting list of illustrative structured proteins that may be used in the fusion proteins described herein is provided in Table 1. In some embodiments, a fragment or variant of any one of the proteins listed in Table 1 may be used. In some embodiments, the structured protein may be an animal protein. For example, in some embodiments, the structured protein may be a mammalian protein. In some embodiments, the structured protein may be a plant protein. For example, the plant protein may be a protein that is not typically expressed in a seed. In some embodiments, the plant protein may be a storage protein, e.g., a protein that acts as a storage reserve for nitrogen, carbon, and/or sulfur. In some embodiments, the plant protein may inhibit one or more proteases. In some embodiments, the structured protein may be a fungal protein. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Structured proteins 
               
            
           
           
               
               
               
               
            
               
                   
                 Protein or Protein 
                   
                 Exemplary Uniprot 
               
               
                 Categories 
                 family 
                 Native Species 
                 Accession No. 
               
               
                   
               
               
                 Mammalian 
                 Alpha-lactalbumin 
                 Bovine ( Bos taurus ) 
                 P00711 
               
               
                   
                 Beta-lactoglobulin 
                 Bovine ( Bos taurus ) 
                 P02754 
               
               
                   
                 Albumin 
                 Bovine ( Bos taurus ) 
                 P02769 
               
               
                   
                 Lysozyme 
                 Bovine ( Bos taurus ) 
                 Q6B411 
               
               
                   
                 Collagen family 
                 Human ( Homo sapiens ) 
                 Q02388, P02452, P08123, 
               
               
                   
                   
                   
                 P02458 
               
               
                   
                 Hemoglobin 
                 Bovine ( Bos taurus ) 
                 P02070 
               
               
                 Avian proteins 
                 Ovalbumin 
                 Chicken ( Gallus gallus ) 
                 P01012 
               
               
                   
                 Ovotransferrin 
                 Chicken ( Gallus gallus ) 
                 P02789 
               
               
                   
                 Ovoglobulin 
                 Chicken ( Gallus gallus ) 
                 I0J170 
               
               
                   
                 Lysozyme 
                 Chicken ( Gallus gallus ) 
                 P00698 
               
               
                 Plant Proteins 
                 Oleosins 
                 Soybean ( Glycine max ) 
                 P29530, P29531 
               
               
                   
                 Leghemoglobin 
                 Soybean ( Glycine max ) 
                 Q41219 
               
               
                   
                 Extensin-like protein 
                 Soybean ( Glycine soja ) 
                 A0A445JU93 
               
               
                   
                 family 
               
               
                   
                 Prolamine 
                 Rice ( Oryza sativa ) 
                 Q0DJ45 
               
               
                   
                 Glutenin 
                 Wheat ( Sorghum bicolor ] 
                 P10388 
               
               
                   
                 Gamma-kafirin 
                 Wheat ( Sorghum bicolor ] 
                 Q41506 
               
               
                   
                 preprotein 
               
               
                   
                 Alpha globulin 
                 Rice ( Oryza sativa ) 
                 P29835 
               
               
                   
                 Basic 7S globulin 
                 Soybean ( Glycine max ) 
                 P13917 
               
               
                   
                 precursor 
               
               
                   
                 2S albumin 
                 Soybean ( Glycine max ) 
                 P19594 
               
               
                   
                 Beta-conglycinins 
                 Soybean ( Glycine max ) 
                 P0DO16, P0DO15, 
               
               
                   
                   
                   
                 P0DO15 
               
               
                   
                 Glycinins 
                 Soybean ( Glycine max ) 
                 P04347, P04776, P04405 
               
               
                   
                 Canein 
                 Sugar cane ( Saccharum   
                 ABP64791.1 
               
               
                   
                   
                   officinarum ) 
               
               
                   
                 Zein 
                 Corn (Zea Mays) 
                 ABP64791.1 
               
               
                   
                 Patatin 
                 Tomato ( Solanum   
                 P07745 
               
               
                   
                   
                   lycopersicum ) 
               
               
                   
                 Kunitz-Trypsin 
                 Soybean ( Glycine max ) 
                 Q39898 
               
               
                   
                 inhibitor 
               
               
                   
                 Bowman-Birk 
                 Soybean ( Glycine max ) 
                 I1MQD2 
               
               
                   
                 inhibitor 
               
               
                   
                 Cystatine 
                 Tomato ( Solanum   
                 Q9SE07 
               
               
                   
                   
                   lycopersicum ) 
               
               
                 Fungal proteins 
                 Hydrophobin I 
                 Fungus ( Trichoderma reesei ) 
                 P52754 
               
               
                   
                 Hydrophobin II 
                 Fungus ( Trichoderma reesei ) 
                 P79073 
               
               
                   
               
            
           
         
       
     
     In some embodiments, the structured protein is an animal protein. In some embodiments, the structured protein is a mammalian protein. For example, the structured protein may be a mammalian protein selected from: β-lactoglobulin, α-lactalbumin, albumin, lysozyme, lactoferrin, lactoperoxidase, hemoglobin, collagen, and an immunoglobulin (e.g., IgA, IgG, IgM, IgE). In some embodiments, the structured mammalian protein is β-lactoglobulin and comprises the sequence of SEQ ID NO: 10, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the structured mammalian protein is β-lactoglobulin and is encoded by the sequence of any one of SEQ ID NO: 9, 11, 12, or 13, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical to any one of SEQ ID NO: 9, 11, 12, or 13. In some embodiments, the structured protein is an avian protein. For example, the structured protein may be an avian protein selected from: ovalbumin, ovotransferrin, lysozyme and ovoglobulin. 
     In some embodiments, the structured protein is a plant protein. For example, the structured protein may be a plant protein selected from: hydrophobin I, hydrophobin II, oleosins, leghemoglobin, extension-like protein family, prolamine, glutenin, gamma-kafirin preprotein, α-globulin, basic 7S globulin precursor, 2S albumin, β-conglycinins, glycinins, canein, zein, patatin, kunitz-trypsin inhibitor, bowman-birk inhibitor, and cystatine. 
     Fusion Proteins 
     Fusion Proteins Comprising an Unstructured Milk Protein and a Structured Animal (e.g., Mammalian) Protein 
     In some embodiments, the fusion proteins described herein comprise (i) an unstructured milk protein, and (ii) a structured animal protein. In some embodiments, the fusion proteins described herein comprise (i) an unstructured milk protein, and (ii) a structured mammalian protein. In some embodiments, the fusion proteins described herein comprise (i) an unstructured milk protein, and (ii) a structured avian protein. In some embodiments, the fusion proteins described herein comprise (i) an unstructured milk protein, and (ii) a structured fungal protein. 
     In some embodiments, the fusion proteins comprise an unstructured milk protein, such as a casein protein. In some embodiments, the fusion proteins comprise an unstructured milk protein selected from α-S1 casein, α-S2 casein, β-casein, and κ-casein. In some embodiments, the fusion proteins comprise an unstructured milk protein isolated or derived from cow ( bos taurus ), goat ( capra hircus ), sheep ( ovis aries ), water buffalo ( bubalus bubalis ), dromedary camel ( camelus dromedaries ), bactrian camel ( camelus bactrianus ), wild yak ( bos mutus ), horse ( equus caballus ), donkey ( equus asinus ), reindeer ( rangifer tarandus ), eurasian elk ( alces alces ), alpaca ( vicugna pacos ), zebu ( bos indicus ), llama ( lama glama ), or human ( homo sapiens ). In some embodiments, the fusion proteins comprise a casein protein (e.g., α-S1 casein, α-S2 casein, β-casein, or κ-casein) from cow ( bos taurus ), goat ( capra hircus ), sheep ( ovis aries ), water buffalo ( bubalus bubalis ), dromedary camel ( camelus dromedaries ), bactrian camel ( camelus bactrianus ), wild yak ( bos mutus ), horse ( equus caballus ), donkey ( equus asinus ), reindeer ( rangifer tarandus ), eurasian elk ( alces alces ), alpaca ( vicugna pacos ), zebu ( bos indicus ), llama ( lama glama ), or human ( homo sapiens ). 
     In some embodiments, the unstructured milk protein is α-S1 casein. In some embodiments, the unstructured milk protein is α-S1 casein and comprises the sequence SEQ ID NO: 8, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the unstructured milk protein is α-S1 casein and comprises the sequence of any one of SEQ ID NO: 99-109, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, the unstructured milk protein is α-S2 casein. In some embodiments, the unstructured milk protein is α-S2 casein and comprises the sequence SEQ ID NO: 84, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the unstructured milk protein is α-S2 casein and comprises the sequence of any one of SEQ ID NO: 110-120, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, the unstructured milk protein is β-casein. In some embodiments, the unstructured milk protein is β-casein and comprises the sequence of SEQ ID NO: 6, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the unstructured milk protein is β-casein and comprises the sequence of any one of SEQ ID NO: 121-133, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, the unstructured milk protein is κ-casein. In some embodiments, the unstructured milk protein is κ-casein and comprises the sequence of SEQ ID NO: 4, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the unstructured milk protein is κ-casein and comprises the sequence of any one of SEQ ID NO: 85-98, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, the unstructured milk protein is para-κ-casein. In some embodiments, the unstructured milk protein is para-κ-casein and comprises the sequence of SEQ ID NO: 2, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, the structured mammalian protein is β-lactoglobulin, α-lactalbumin, albumin, lysozyme, lactoferrin, lactoperoxidase, hemoglobin, collagen, or an immunoglobulin (e.g., IgA, IgG, IgM, or IgE). In some embodiments, the structured avian protein is ovalbumin, ovotransferrin, lysozyme or ovoglobulin. 
     In some embodiments, the structured mammalian protein is β-lactoglobulin. In some embodiments, the structured mammalian protein is β-lactoglobulin and comprises the sequence of SEQ ID NO: 10, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, a fusion protein comprises a casein protein (e.g., κ-casein, para-κ-casein, β-casein, or α-S1 casein) and β-lactoglobulin. In some embodiments, a fusion protein comprises κ-casein and β-lactoglobulin (see, e.g.,  FIG.  3   ,  FIG.  8   ,  FIG.  10 A- 10 B ). In some embodiments, a fusion protein comprises para-κ-casein and β-lactoglobulin (see, e.g.,  FIG.  6   ,  FIG.  7   ,  FIG.  10 A- 10 B ). In some embodiments, a fusion protein comprises β-casein and β-lactoglobulin. In some embodiments, a fusion protein comprises α-S1 casein and β-lactoglobulin. 
     In some embodiments, a plant-expressed recombinant fusion protein comprises κ-casein, or fragment thereof; and β-lactoglobulin, or fragment thereof. In some embodiments, the fusion protein comprises, in order from N-terminus to C-terminus, the κ-casein and the β-lactoglobulin. 
     Fusion Protein Comprising an Unstructured Milk Protein and a Structured Plant Protein 
     In some embodiments, the fusion proteins described herein comprise (i) an unstructured milk protein, and (ii) a structured plant protein. In some embodiments, the unstructured milk protein is a casein protein, such as α-S1 casein, α-S2 casein, β-casein, or κ-casein. In some embodiments, the plant protein is selected from the group consisting of: hydrophobin I, hydrophobin II, oleosins, leghemoglobin, extension-like protein family, prolamine, glutenin, gamma-kafirin preprotein, α-globulin, basic 7S globulin precursor, 2S albumin, β-conglycinins, glycinins, canein, zein, patatin, kunitz-trypsin inhibitor, bowman-birk inhibitor, and cystatine. 
     Fusion Protein Structure 
     The fusion proteins described herein may have various different structures, in order to increase expression and/or accumulation in a plant or other host organism or cell. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, an unstructured milk protein and a structured animal (e.g., mammalian or avian) protein. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, a structured animal (e.g., mammalian or avian) protein and a milk protein. For example, in some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus κ-casein and β-lactoglobulin. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus β-lactoglobulin and κ-casein. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, para-κ-casein and β-lactoglobulin. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, β-lactoglobulin and para-κ-casein. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, β-casein and β-lactoglobulin. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, β-lactoglobulin and β-casein. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, α-S1 casein and β-lactoglobulin. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, β-lactoglobulin and α-S1 casein. 
     In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, an unstructured milk protein and a structured plant protein. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, a structured plant protein and a milk protein. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, a casein protein and a structured plant protein. In some embodiments, a fusion protein comprises, in order from N-terminus to C-terminus, a structured plant protein and a casein protein. 
     In some embodiments, a fusion protein comprises a protease cleavage site. For example, in some embodiments, the fusion protein comprises an endoprotease, endopeptidase, and/or endoproteinase cleavage site. In some embodiments, the fusion protein comprises a rennet cleavage site. In some embodiments, the fusion protein comprises a chymosin cleavage site. In some embodiments, the fusion protein comprises a trypsin cleavage site. 
     The protease cleavage site may be located between the unstructured milk protein and the structured animal (e.g., mammalian or avian) protein, or between the unstructured milk protein and the structured plant protein, such that cleavage of the protein at the protease cleavage site will separate the unstructured milk protein from the structured animal (e.g., mammalian or avian) or plant protein. 
     In some embodiments, the protease cleavage site may be contained within the sequence of either the milk protein or the structured animal (e.g., mammalian or animal) or plant protein. In some embodiments, the protease cleavage site may be added separately, for example, between the two proteins. 
     In some embodiments, a fusion protein comprises a linker between the unstructured milk protein and the structured animal (e.g., mammalian or avian) protein, or between the unstructured milk protein and the structured plant protein. In some embodiments, the linker may comprise a peptide sequence recognizable by an endoprotease. In some embodiments, the linker may comprise a protease cleavage site. In some embodiments, the linker may comprise a self-cleaving peptide, such as a 2A peptide. 
     In some embodiments, a fusion protein may comprise a signal peptide. The signal peptide may be cleaved from the fusion protein, for example, during processing or transport of the protein within the cell. In some embodiments, the signal peptide is located at the N-terminus of the fusion protein. In some embodiments, the signal peptide is located at the C-terminus of the fusion protein. 
     In some embodiments, the signal peptide is selected from the group consisting of GmSCB1, StPat21, 2Sss, Sig2, Sig12, Sig8, Sig10, Sig11, and Coixss. In some embodiments, the signal peptide is Sig10 and comprises SEQ ID NO: 15, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the signal peptide is Sig2 and comprises SEQ ID NO: 17, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 71. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 73. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 75. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 77. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 79. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 81. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 135. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 137. 
     In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 71, with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more amino acid substitutions. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 73, with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more amino acid substitutions. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 75, with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more amino acid substitutions. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 77, with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more amino acid substitutions. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 79, with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more amino acid substitutions. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 81, with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more amino acid substitutions. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 135, with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more amino acid substitutions. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 137, with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more amino acid substitutions. 
     In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 71, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 73, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 75, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 77, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 79, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 81, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 135, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the fusion protein comprises the sequence of SEQ ID NO: 137, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, the fusion proteins have a molecular weight in the range of about 1 kDa to about 500 kDa, about 1 kDa to about 250 kDa, about 1 to about 100 kDa, about 10 to about 50 kDa, about 1 to about 10 kDa, about 10 to about 200 kDa, about 30 to about 150 kDa, about 30 kDa to about 50 kDa, or about 20 to about 80 kDa. 
     Nucleic Acids Encoding Fusion Proteins and Vectors Comprising the Same 
     Also provided herein are nucleic acids encoding the fusion proteins of the disclosure, for example fusion proteins comprising an unstructured milk protein and a structured animal (e.g., mammalian or avian) or plant protein. In some embodiments, the nucleic acids are DNAs. In some embodiments, the nucleic acids are RNAs. 
     In some embodiments, a nucleic acid comprises a sequence encoding a fusion protein. In some embodiments, a nucleic acid comprises a sequence encoding a fusion protein, which is operably linked to a promoter. In some embodiments, a nucleic acid comprises, in order from 5′ to 3′, a promoter, a 5′ untranslated region (UTR), a sequence encoding a fusion protein, and a terminator. 
     The promoter may be a plant promoter. A “plant promoter” is a promoter capable of initiating transcription in plant cells. Examples of promoters under developmental control include promoters that preferentially initiate transcription in certain organs, such as leaves, roots, flowers, seeds and tissues such as fibers, xylem vessels, tracheids, or sclerenchyma. Such promoters are referred to as “tissue-preferred.” Promoters which initiate transcription only in certain tissue are referred to as “tissue-specific.” A “cell-type” specific promoter primarily drives expression in certain cell types in one or more organs, for example, vascular cells in leaves, roots, flowers, or seeds. An “inducible” promoter is a promoter which is under environmental control. Examples of environmental conditions that may affect transcription by inducible promoters include anaerobic conditions or the presence of light. Tissue-specific, tissue-preferred, cell-type specific, and inducible promoters constitute the class of “non-constitutive” promoters. A “constitutive” promoter is a promoter which is active under most environmental conditions. 
     In some embodiments, the promoter is a plant promoter derived from, for example soybean, lima bean, Arabidopsis, tobacco, rice, maize, barley, sorghum, wheat, pea, and/or oat. In some embodiments, the promoter is a constitutive or an inducible promoter. Exemplary constitutive promoters include, but are not limited to, the promoters from plant viruses such as the 35S promoter from CaMV and the promoters from such genes as rice actin; ubiquitin; pEMU; MAS and maize H3 histone. In some embodiments, the constitutive promoter is the ALS promoter, Xbal/Ncol fragment 5′ to the  Brassica napus  ALS3 structural gene (or a nucleotide sequence similarity to said Xbal/Ncol fragment). 
     In some embodiments, the promoter is a plant tissue-specific or tissue-preferential promoter. In some embodiments, the promoter is isolated or derived from a soybean gene. Illustrative soybean tissue-specific promoters include AR-Pro1, AR-Pro2, AR-Pro3, AR-Pro4, AR-Pro5, AR-Pro6, AR-Pro7, AR-Pro8, and AR-Pro9. 
     In some embodiments, the plant is a seed-specific promoter. In some embodiments, the seed-specific promoter is selected from the group consisting of PvPhas, BnNap, AtOle1, GmSeed2, GmSeed3, GmSeed5, GmSeed6, GmSeed7, GmSeed8, GmSeed10, GmSeed11, GmSeed12, pBCON, GmCEP1-L, GmTHIC, GmBg7S1, GmGRD, GmOLEA, GmOLER, Gm2S-1, and GmBBld-II. In some embodiments, the seed-specific promoter is PvPhas and comprises the sequence of SEQ ID NO: 18, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the seed-specific promoter is GmSeed2 and comprises the sequence of SEQ ID NO: 19, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. In some embodiments, the promoter is a Cauliflower Mosaic Virus (CaMV) 35S promoter. 
     In some embodiments, the promoter is a soybean polyubiquitin (Gmubi) promoter, a soybean heat shock protein 90-like (GmHSP90L) promoter, a soybean Ethylene Response Factor (GmERF) promoter. In some embodiments, the promoter is a constitutive soybean promoter derived from GmScreamM1, GmScreamM4, GmScreamM8 genes or GmubiXL genes. 
     In some embodiments, the 5′ UTR is selected from the group consisting of Arc5′UTR and glnB1UTR. In some embodiments, the 5′ untranslated region is Arc5′UTR and comprises the sequence of SEQ ID NO: 20, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, the terminator sequence is isolated or derived from a gene encoding Nopaline synthase, Arc5-1, an Extensin, Rb7 matrix attachment region, a Heat shock protein, Ubiquitin 10, Ubiquitin 3, and M6 matrix attachment region. In some embodiments, the terminator sequence is isolated or derived from a Nopaline synthase gene and comprises the sequence of SEQ ID NO: 22, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments, the nucleic acid comprises a 3′ UTR. For example, the 3′ untranslated region may be Arc5-1 and comprise SEQ ID NO: 21, or a sequence at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% identical thereto. 
     In some embodiments the nucleic acid comprises a gene encoding a selectable marker. One illustrative selectable marker gene for plant transformation is the neomycin phosphotransferase II (nptll) gene, isolated from transposon Tn5, which, when placed under the control of plant regulatory signals, confers resistance to kanamycin. Another exemplary marker gene is the hygromycin phosphotransferase gene which confers resistance to the antibiotic hygromycin. In some embodiments, the selectable marker is of bacterial origin and confers resistance to antibiotics such as gentamycin acetyl transferase, streptomycin phosphotransferase, and aminoglycoside-3′-adenyl transferase, the bleomycin resistance determinant. In some embodiments, the selectable marker genes confer resistance to herbicides such as glyphosate, glufosinate or bromoxynil. In some embodiments, the selectable marker is mouse dihydrofolate reductase, plant 5-enolpyruvylshikimate-3-phosphate synthase and plant acetolactate synthase. In some embodiments, the selectable marker is acetolactate synthase (e.g., AtCsr1.2). 
     In some embodiments, a nucleic acid comprises an endoplasmic reticulum retention signal. For example, in some embodiments, a nucleic acid comprises a KDEL sequence (SEQ ID NO: 23). In some embodiments, the nucleic acid may comprise an endoplasmic reticulum retention signal selected from any one of SEQ ID NO: 23-70. 
     Shown in Table 2 are exemplary promoters, 5′ UTRs, signal peptides, and terminators that may be used in the nucleic acids of the disclosure. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Promoters, 5′ UTRs, signal peptides and terminators 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Illustrative 
               
               
                   
                   
                   
                   
                 Accession No. 
               
               
                 Type 
                 Name 
                 Description 
                 Native Species 
                 (Glyma, GenBank) 
               
               
                   
               
               
                 Promoter 
                 PvPhas 
                 Phaseolin-1 (aka β-phaseolin) 
                 Common bean 
                 J01263.1 
               
               
                   
                   
                   
                 ( Phaseolus vulgaris ) 
               
               
                   
                 BnNap 
                 Napin-1 
                 Rapeseed ( Brassica   
                 J02798.1 
               
               
                   
                   
                   
                   napus ) 
               
               
                   
                 AtOle1 
                 Oleosin-1 (Ole1) 
                   Arabidopsis  ( Arabidopsis   
                 X62353.1, 
               
               
                   
                   
                   
                   thaliana ) 
                 AT4G25140 
               
               
                   
                 GmSeed2 
                 Gy1 (Glycinin 1) 
                 Soybean ( Glycine max ) 
                 Glyma.03G163500 
               
               
                   
                 GmSeed3 
                 cysteine protease 
                 Soybean ( Glycine max ) 
                 Glyma.08G116300 
               
               
                   
                 GmSeed5 
                 Gy5 (Glycinin 5) 
                 Soybean ( Glycine max ) 
                 Glyma.13G123500 
               
               
                   
                 GmSeed6 
                 Gy4 (Glycinin 4) 
                 Soybean ( Glycine max ) 
                 Glyma.10G037100 
               
               
                   
                 GmSeed7 
                 Kunitz trypsin protease 
                 Soybean ( Glycine max ) 
                 Glyma.01G095000 
               
               
                   
                   
                 inhibitor 
               
               
                   
                 GmSeed8 
                 Kunitz trypsin protease 
                 Soybean ( Glycine max ) 
                 Glyma.08G341500 
               
               
                   
                   
                 inhibitor 
               
               
                   
                 GmSeed10 
                 Legume Lectin Domain 
                 Soybean ( Glycine max ) 
                 Glyma.02G012600 
               
               
                   
                 GmSeed11 
                 β-conglycinin a subunit 
                 Soybean ( Glycine max ) 
                 Glyma.20G148400 
               
               
                   
                 GmSeed12 
                 β-conglycinin a′ subunit 
                 Soybean ( Glycine max ) 
                 Glyma.10G246300 
               
               
                   
                 pBCON 
                 β-conglycinin β subunit 
                 Soybean ( Glycine max ) 
                 Glyma.20G148200 
               
               
                   
                 GmCEP1-L 
                 KDEL-tailed cysteine 
                 Soybean ( Glycine max ) 
                 Glyma06g42780 
               
               
                   
                   
                 endopeptidase CEP1-like 
               
               
                   
                 GmTHIC 
                 phosphomethylpyrimidine 
                 Soybean ( Glycine max ) 
                 Glyma11g26470 
               
               
                   
                   
                 synthase 
               
               
                   
                 GmBg7S1 
                 Basic 7S globulin precursor 
                 Soybean ( Glycine max ) 
                 Glyma03g39940 
               
               
                   
                 GmGRD 
                 glucose and ribitol 
                 Soybean ( Glycine max ) 
                 Glyma07g38790 
               
               
                   
                   
                 dehydrogenase-like 
               
               
                   
                 GmOLEA 
                 Oleosin isoform A 
                 Soybean ( Glycine max ) 
                 Glyma.19g063400 
               
               
                   
                 GmOLEB 
                 Oleosin isoform B 
                 Soybean ( Glycine max ) 
                 Glyma.16g071800 
               
               
                   
                 Gm2S-1 
                 2S albumin 
                 Soybean ( Glycine max ) 
                 Glyma13g36400 
               
               
                   
                 GmBBId-II 
                 Bowman-Birk protease 
                 Soybean ( Glycine max ) 
                 Glyma16g33400 
               
               
                   
                   
                 inhibitor 
               
               
                 5′UTR 
                 Arc5′UTR 
                 arc5-1 gene 
                 
                   Phaseolus vulgaris 
                 
                 J01263.1 
               
               
                   
                 glnB1UTR 
                 65 bp of native glutamine 
                 Soybean ( Glycine max ) 
                 AF301590.1 
               
               
                   
                   
                 synthase 
               
               
                 Signal peptide 
                 GmSCB1 
                 Seed coat BURP domain 
                 Soybean ( Glycine max ) 
                 Glyma07g28940.1 
               
               
                   
                   
                 protein 
               
               
                   
                 StPat21 
                 Patatin 
                 Tomato ( Solanum   
                 CAA27588 
               
               
                   
                   
                   
                   lycopersicum ) 
               
               
                   
                 2Sss 
                 2S albumin 
                 Soybean ( Glycine max ) 
                 Glyma13g36400 
               
               
                   
                 Sig2 
                 Glycinin G1 N-terminal 
                 Soybean ( Glycine max ) 
                 Glyma.03G163500 
               
               
                   
                   
                 peptide 
               
               
                   
                 Sig12 
                 Beta-conglycinin alpha prime 
                 Soybean ( Glycine max ) 
                 Glyma.10G246300 
               
               
                   
                   
                 subunit N-terminal peptide 
               
               
                   
                 Sig8 
                 Kunitz trypsin inhibitor N- 
                 Soybean ( Glycine max ) 
                 Glyma.08G341500 
               
               
                   
                   
                 terminal peptide 
               
               
                   
                 Sig10 
                 Lectin N-terminal peptide 
                 Soybean ( Glycine max ) 
                 Glyma.02G012600 
               
               
                   
                   
                 from  Glycine max   
               
               
                   
                 Sig11 
                 Beta-conglycinin alpha 
                 Soybean ( Glycine max ) 
                 Glyma.20G148400 
               
               
                   
                   
                 subunit N-terminal peptide 
               
               
                   
                 Coixss 
                 Alpha-coixin N-terminal 
                   Coix lacryma -job 
               
               
                   
                   
                 peptide from  Coix lacryma - 
               
               
                   
                   
                 job 
               
               
                   
                 KDEL 
                 C-terminal amino acids of 
                 
                   Phaseolus vulgaris 
                 
               
               
                   
                   
                 sulfhydryl endopeptidase 
               
               
                 Terminator 
                 NOS 
                 Nopaline synthase gene 
                 
                   Agrobacterium 
                 
               
               
                   
                   
                 termination sequence 
                 
                   tumefaciens 
                 
               
               
                   
                 ARC 
                 arc5-1 gene termination 
                 
                   Phaseolus vulgaris 
                 
                 J01263.1 
               
               
                   
                   
                 sequence 
               
               
                   
                 EU 
                 Extensin termination sequence 
                 
                   Nicotiana tabacum 
                 
               
               
                   
                 Rb7 
                 Rb7 matrix attachment region 
                 
                   Nicotiana tabacum 
                 
               
               
                   
                   
                 termination sequence 
               
               
                   
                 HSP or 
                 Heat shock termination 
                 
                   Arabidopsis thaliana 
                 
               
               
                   
                 AtHSP 
                 sequence 
               
               
                   
                 AtUbi10 
                 Ubiquitin 10 termination 
                 
                   Arabidopsis thaliana 
                 
               
               
                   
                   
                 sequence 
               
               
                   
                 Stubi3 
                 Ubiquitin 3 termination 
                 
                   Solanum tuberosum 
                 
               
               
                   
                 TM6 
                 M6 matrix attachment region 
                 
                   Nicotiana tabacum 
                 
               
               
                   
                   
                 termination sequence 
               
               
                   
               
            
           
         
       
     
     Illustrative nucleic acids of the disclosure are provided in  FIG.  1 A- 1 P . In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding an unstructured milk protein, a sequence encoding a structured mammalian protein, an endoplasmic reticulum retention signal, and a terminator (See, e.g.,  FIG.  1 A ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding an unstructured milk protein, a sequence encoding a linker, a sequence encoding a structured mammalian protein, an endoplasmic reticulum retention signal, and a terminator (See, e.g.,  FIG.  1 B ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding an unstructured milk protein, a sequence encoding a linker, a sequence encoding a structured mammalian protein, and a terminator (See, e.g.,  FIG.  1 C ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding an unstructured milk protein, a sequence encoding a structured mammalian protein, and a terminator (See, e.g.,  FIG.  1 D ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a structured mammalian protein, a sequence encoding an unstructured milk protein, an endoplasmic reticulum retention signal, and a terminator (See, e.g.,  FIG.  1 E ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a structured mammalian protein, a sequence encoding a linker, a sequence encoding an unstructured milk protein, an endoplasmic reticulum retention signal, and a terminator (See, e.g.,  FIG.  1 F ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a structured mammalian protein, a sequence encoding a linker, a sequence encoding an unstructured milk protein, and a terminator (See, e.g.,  FIG.  1 G ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a structured mammalian protein, a sequence encoding an unstructured milk protein, and a terminator (See, e.g.,  FIG.  1 H ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a signal peptide, a sequence encoding an unstructured milk protein, a sequence encoding a structured mammalian protein, an endoplasmic reticulum retention signal, and a terminator (See, e.g.,  FIG.  1 I ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a signal peptide, a sequence encoding an unstructured milk protein, a sequence encoding a linker, a sequence encoding a structured mammalian protein, an endoplasmic reticulum retention signal, and a terminator (See, e.g.,  FIG.  1 J ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a signal peptide, a sequence encoding an unstructured milk protein, a sequence encoding a linker, a sequence encoding a structured mammalian protein, and a terminator (See, e.g.,  FIG.  1 K ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a signal peptide, a sequence encoding an unstructured milk protein, a sequence encoding a structured mammalian protein, and a terminator (See, e.g.,  FIG.  1 L ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a signal peptide, a sequence encoding a structured mammalian protein, a sequence encoding an unstructured milk protein, an endoplasmic reticulum retention signal, and a terminator (See, e.g.,  FIG.  1 M ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a signal peptide, a sequence encoding a structured mammalian protein, a sequence encoding a linker, a sequence encoding an unstructured milk protein, an endoplasmic reticulum retention signal, and a terminator (See, e.g.,  FIG.  1 N ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a signal peptide, a sequence encoding a structured mammalian protein, a sequence encoding a linker, a sequence encoding an unstructured milk protein, and a terminator (See, e.g.,  FIG.  1 O ). In some embodiments a nucleic acid comprises, from 5′ to 3′, a promoter, a 5′UTR, a sequence encoding a signal peptide, a sequence encoding a structured mammalian protein, a sequence encoding an unstructured milk protein, and a terminator (See, e.g.,  FIG.  1 P ). 
     In some embodiments, the nucleic acid comprises an expression cassette comprising a OKC1-T:OLG1 (Optimized Kappa Casein version 1:beta-lactoglobulin version 1) fusion driven by PvPhas promoter fused with arc5′UTR:sig10, followed by the ER retention signal (KDEL) and the 3′UTR of the arc5-1 gene, “arc-terminator” (See, e.g.,  FIG.  3   ). In some embodiments, the nucleic acid comprises SEQ ID NO: 72. 
     In some embodiments, the nucleic acid comprises an expression cassette comprising a OBC-T2:FM:OLG1 (Optimized Beta Casein Truncated version 2:Chymosin cleavage site:beta-lactoglobulin version 1) fusion driven by PvPhas promoter fused with arc5′UTR:sig10, followed by the 3′UTR of the arc5-1 gene, “arc-terminator” (See, e.g.,  FIG.  4   ). In some embodiments, the nucleic acid comprises SEQ ID NO: 74. The Beta Casein is “truncated” in that the bovine secretion signal is removed, and replaced with a plant targeting signal. 
     In some embodiments, the nucleic acid comprises an expression cassette comprising a OaS1-T:FM:OLG1 (Optimized Alpha S1 Casein Truncated version 1:Chymosin cleavage site:beta-lactoglobulin version 1) fusion driven by PvPhas promoter fused with arc5′UTR:sig10, followed by the 3′UTR of the arc5-1 gene, “arc-terminator” (See, e.g.,  FIG.  5   ). In some embodiments, the nucleic acid comprises SEQ ID NO: 76. The Alpha S1 is “truncated” in that the bovine secretion signal is removed, and replaced with a plant targeting signal. 
     In some embodiments, the nucleic acid comprises an expression cassette comprising a para-OKC1-T:FM:OLG1:KDEL (Optimized paraKappa Casein version 1:Chymosin cleavage site:beta-lactoglobulin version 1) fusion driven by PvPhas promoter fused with arc5′UTR:sig 10, followed by the ER retention signal (KDEL) and the 3′UTR of the arc5-1 gene, “arc-terminator” (See, e.g.,  FIG.  6   ). In some embodiments, the nucleic acid comprises SEQ ID NO: 78. 
     In some embodiments, the nucleic acid comprises an expression cassette comprising a para-OKC1-T:FM:OLG1 (Optimized paraKappa Casein version 1:Chymosin cleavage site:beta-lactoglobulin version 1) fusion driven by PvPhas promoter fused with arc5′UTR:sig 10, followed by the 3′UTR of the arc5-1 gene, “arc-terminator” (See, e.g.,  FIG.  7   ). In some embodiments, the nucleic acid comprises SEQ ID NO: 80. 
     In some embodiments, the nucleic acid comprises an expression cassette comprising a OKC1-T-OLG1 (Optimized Kappa Casein version 1:beta-lactoglobulin version 1) fusion that is driven by the promoter and signal peptide of glycinin 1 (GmSeed2:sig2) followed by the ER retention signal (KDEL) and the nopaline synthase gene termination sequence (nos term) (See, e.g.,  FIG.  8   ). In some embodiments, the nucleic acid comprises SEQ ID NO: 82. 
     In some embodiments, a nucleic acid encoding a fusion protein comprises the sequence of any one of SEQ ID NO: 72, 74, 76, 78, 80, 82, 134, or 136. 
     In some embodiments, the nucleic acids are codon optimized for expression in a host cell. Codon optimization is a process used to improve gene expression and increase the translational efficiency of a gene of interest by accommodating codon bias of the host organism (i.e., the organism in which the gene is expressed). Codon-optimized mRNA sequences that are produced using different programs or approaches can vary because different codon optimization strategies differ in how they quantify codon usage and implement codon changes. Some approaches use the most optimal (frequently used) codon for all instances of an amino acid, or a variation of this approach. Other approaches adjust codon usage so that it is proportional to the natural distribution of the host organism. These approaches include codon harmonization, which endeavors to identify and maintain regions of slow translation thought to be important for protein folding. Alternative approaches involve using codons thought to correspond to abundant tRNAs, using codons according to their cognate tRNA concentrations, selectively replacing rare codons, or avoiding occurrences of codon-pairs that are known to translate slowly. In addition to approaches that vary in the extent to which codon usage is considered as a parameter, there are hypothesis-free approaches that do not consider this parameter. Algorithms for performing codon optimization are known to those of skill in the art and are widely available on the Internet. 
     In some embodiments the nucleic acids are codon optimized for expression in a plant species. The plant species may be, for example, a monocot or a dicot. In some embodiments, the plant species is a dicot species selected from soybean, lima bean, Arabidopsis, tobacco, rice, maize, barley, sorghum, wheat and/or oat. In some embodiments, the plant species is soybean. 
     The nucleic acids of the disclosure may be contained within a vector. The vector may be, for example, a viral vector or a non-viral vector. In some embodiments, the non-viral vector is a plasmid, such as an  Agrobacterium  Ti plasmid. In some embodiments, the non-viral vector is a lipid nanoparticle. 
     In some embodiments, a vector comprises a nucleic acid encoding a recombinant fusion protein, wherein the recombinant fusion protein comprises: (i) an unstructured milk protein, and (ii) a structured animal (e.g., mammalian or avian) protein. In some embodiments, the vector is an  Agrobacterium  Ti plasmid. 
     In some embodiments, a method for expressing a fusion protein in a plant comprises contacting the plant with a vector of the disclosure. In some embodiments, the method comprises maintaining the plant or part thereof under conditions in which the fusion protein is expressed. 
     Plants Expressing Fusion Proteins 
     Also provided herein are transgenic plants expressing one or more fusion proteins of the disclosure. In some embodiments, the transgenic plants stably express the fusion protein. In some embodiments, the transgenic plants stably express the fusion protein in the plant in an amount of at least 1% per the total protein weight of the soluble protein extractable from the plant. For example, the transgenic plants may stably express the fusion protein in an amount of at least 1%, at least 1.5%, at least 2%, at least 2.5%, at least 3%, at least 3.5%, at least 4%, at least 4.5%, at least 5%, at least 5.5%, at least 6%, at least 6.5%, at least 7%, at least 7.5%, at least 8%, at least 8.5%, at least 9%, at least 9.5%, at least 10%, at least 10.5%, at least 11%, at least 11.5%, at least 12%, at least 12.5%, at least 13%, at least 13.5%, at least 14%, at least 14.5%, at least 15%, at least 15.5%, at least 16%, at least 16.5%, at least 17%, at least 17.5%, at least 18%, at least 18.5%, at least 19%, at least 19.5%, at least 20%, or more of total protein weight of soluble protein extractable from the plant. 
     In some embodiments, the transgenic plants stably express the fusion protein in an amount of less than about 1% of the total protein weight of soluble protein extractable from the plant. In some embodiments, the transgenic plants stably express the fusion protein in the range of about 1% to about 2%, about 3% to about 4%, about 4% to about 5%, about 5% to about 6%, about 6% to about 7%, about 7% to about 8%, about 8% to about 9%, about 9% to about 10%, about 10% to about 11%, about 11% to about 12%, about 12% to about 13%, about 13% to about 14%, about 14% to about 15%, about 15% to about 16%, about 16% to about 17%, about 17%, to about 18%, about 18% to about 19%, about 19% to about 20%, or more than about 20% of the total protein weight of soluble protein extractable from the plant. 
     In some embodiments, the transgenic plant stably express the fusion protein in an amount in the range of about 0.5% to about 3%, about 1% to about 4%, about 1% to about 5%, about 2% to about 5%, about 1% to about 10%, about 2% to about 10%, about 3% to about 10%, about 5 to about 12%, about 4% to about 10%, or about 5% to about 10%, about 4% to about 8%, about 5% to about 15%, about 5% to about 18%, about 10% to about 20%, or about 1% to about 20% of the total protein weight of soluble protein extractable from the plant. 
     In some embodiments, the fusion protein is expressed at a level at least 2-fold higher than an unstructured milk protein expressed individually in a plant. For example, in some embodiments, the fusion protein is expressed at a level at least 2-fold, at least 2.5-fold, at least 3-fold, at least 3.5-fold, at least 4-fold, at least 4.5-fold, at least 5-fold, at least 5.5-fold, at least 6-fold, at least 7-fold, at least 7.5-fold, at least 8-fold, at least 8.5-fold, at least 9-fold, at least 9.5-fold, at least 10-fold, at least 25-fold, at least 50-fold, or at least 100-fold higher than an unstructured milk protein expressed individually in a plant. 
     In some embodiments, the fusion protein accumulates in the plant at least 2-fold higher than an unstructured milk protein expressed without the structured animal (e.g., mammalian or avian) protein. For example, in some embodiments, the fusion protein accumulates in the plant at least 2-fold, at least 2.5-fold, at least 3-fold, at least 3.5-fold, at least 4-fold, at least 4.5-fold, at least 5-fold, at least 5.5-fold, at least 6-fold, at least 7-fold, at least 7.5-fold, at least 8-fold, at least 8.5-fold, at least 9-fold, at least 9.5-fold, at least 10-fold, at least 25-fold, at least 50-fold, or at least 100-fold higher than an unstructured milk protein expressed without the structured animal protein. 
     In some embodiments, a stably transformed plant comprises in its genome: a recombinant DNA construct encoding a fusion protein, wherein the fusion protein comprises (i) an unstructured milk protein, and (ii) a structured animal (e.g., mammalian or avian) protein. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 1% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 2% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 3% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 4% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 5% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 6% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 7% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 8% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 9% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 10% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 11% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 12% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 13% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 14% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 15% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 16% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 17% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 18% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 19% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the fusion protein is stably expressed in the plant in an amount of 20% or higher per the total protein weight of the soluble protein extractable from the plant. 
     In some embodiments, a stably transformed plant comprises in its genome: a recombinant DNA construct encoding a fusion protein, wherein the fusion protein comprises from N-terminus to C-terminus, the unstructured milk protein and the animal (e.g., mammalian or avian) protein. In some embodiments, the fusion protein comprises, from N-terminus to C-terminus, the structured animal (e.g., mammalian or avian) protein and the milk protein. 
     In some embodiments, a stably transformed plant comprises in its genome: a recombinant DNA construct encoding a fusion protein, wherein the fusion protein comprises an unstructured milk protein such as a casein protein. In In some embodiments, a stably transformed plant comprises in its genome: a recombinant DNA construct encoding a fusion protein, wherein the fusion protein comprises an unstructured milk protein selected from α-S1 casein, α-S2 casein, β-casein, and κ-casein. In some embodiments, the unstructured milk protein is α-S1 casein. In some embodiments, the unstructured milk protein is α-S1 casein and comprises the sequence SEQ ID NO: 8, or a sequence at least 90% identical thereto. In some embodiments, the unstructured milk protein is α-S2 casein. In some embodiments, the unstructured milk protein is α-S2 casein and comprises the sequence SEQ ID NO: 84, or a sequence at least 90% identical thereto. In some embodiments, the unstructured milk protein is β-casein. In some embodiments, the unstructured milk protein is β-casein and comprises the sequence of SEQ ID NO: 6, or a sequence at least 90% identical thereto. In some embodiments, the unstructured milk protein is κ-casein. In some embodiments, the unstructured milk protein is κ-casein and comprises the sequence of SEQ ID NO: 4, or a sequence at least 90% identical thereto. In some embodiments, the unstructured milk protein is para-κ-casein. In some embodiments, the unstructured milk protein is para-κ-casein and comprises the sequence of SEQ ID NO: 2, or a sequence at least 90% identical thereto. 
     In some embodiments, a stably transformed plant comprises in its genome: a recombinant DNA construct encoding a fusion protein, wherein the fusion protein comprises a structured mammalian protein selected from β-lactoglobulin, α-lactalbumin, albumin, lysozyme, lactoferrin, lactoperoxidase, hemoglobin, collagen, and an immunoglobulin (e.g., IgA, IgG, IgM, or IgE). In some embodiments, the structured mammalian protein is β-lactoglobulin. In some embodiments, the structured mammalian protein is β-lactoglobulin and comprises the sequence of SEQ ID NO: 10, or a sequence at least 90% identical thereto. In some embodiments, a stably transformed plant comprises in its genome: a recombinant DNA construct encoding a fusion protein, wherein the fusion protein comprises a structured avian protein selected from lysozyme, ovalbumin, ovotransferrin, and ovoglobulin. 
     In some embodiments, a stably transformed plant comprises in its genome: a recombinant DNA construct encoding a fusion protein, wherein the fusion protein comprises a casein protein and β-lactoglobulin. In some embodiments, a stably transformed plant comprises in its genome: a recombinant DNA construct encoding a fusion protein, wherein the fusion protein comprises κ-casein and β-lactoglobulin. In some embodiments, the fusion protein comprises para-κ-casein and β-lactoglobulin. In some embodiments, the fusion protein comprises β-casein and β-lactoglobulin. In some embodiments, the fusion protein comprises α-S1 casein and β-lactoglobulin. 
     In some embodiments, a stably transformed plant comprises in its genome: a recombinant DNA construct encoding a fusion protein; wherein the fusion protein comprises (1) κ-casein, and (ii) β-lactoglobulin. In some embodiments; and wherein the fusion protein is stably expressed in the plant in an amount of 1% or higher per the total protein weight of the soluble protein extractable from the plant. 
     In some embodiments, the stably transformed plant is a monocot. For example, in some embodiments, the plant may be a monocot selected from turf grass, maize (corn), rice, oat, wheat, barley, sorghum, orchid, iris, lily, onion, palm, and duckweed. 
     In some embodiments, the stably transformed plant is a dicot. For example, in some embodiments, the plant may be a dicot selected from Arabidopsis, tobacco, tomato, potato, sweet potato, cassava, alfalfa, lima bean, pea, chick pea, soybean, carrot, strawberry, lettuce, oak, maple, walnut, rose, mint, squash, daisy, Quinoa, buckwheat, mung bean, cow pea, lentil, lupin, peanut, fava bean, French beans (i.e., common beans), mustard, or cactus. In some embodiments, the plant is a soybean (Glycine max). 
     In some embodiments, the plant is a non-vascular plant selected from moss, liverwort, hornwort or algae. In some embodiments, the plant is a vascular plant reproducing from spores (e.g., a fern). 
     In some embodiments, the recombinant DNA construct is codon-optimized for expression in the plant. For example, in some embodiments, the recombinant DNA construct is codon-optimized for expression in a soybean plant. 
     The transgenic plants described herein may be generated by various methods known in the art. For example, a nucleic acid encoding a fusion protein may be contacted with a plant, or a part thereof, and the plant may then be maintained under conditions wherein the fusion protein is expressed. In some embodiments, the nucleic acid is introduced into the plant, or part thereof, using one or more methods for plant transformation known in the art, such as  Agrobacterium -mediated transformation, particle bombardment-medicated transformation, electroporation, and microinjection. 
     In some embodiments, a method for stably expressing a recombinant fusion protein in a plant comprises (i) transforming a plant with a plant transformation vector comprising an expression cassette comprising: a sequence encoding a fusion protein, wherein the fusion protein comprises an unstructured milk protein, and a structured animal (e.g., mammalian or avian) protein; and (ii) growing the transformed plant under conditions wherein the recombinant fusion protein is expressed. In some embodiments, the recombinant fusion protein is expressed in an amount of 1% or higher per the total protein weight of the soluble protein extractable from the plant. In some embodiments, the unstructured milk protein is κ-casein. In some embodiments, the structured mammalian protein is β-lactoglobulin. In some embodiments, the unstructured milk protein is κ-casein and the structured mammalian protein is β-lactoglobulin. 
     Food Compositions Comprising a Fusion Protein 
     The fusion proteins and transgenic plants described herein may be used to prepare food compositions. The fusion protein may be used directly to prepare the food composition (i.e., in the form of a fusion protein), or the fusion protein may first be separated into its constituent proteins. For example, in some embodiments, a food composition may comprise either (i) a fusion protein, (ii) an unstructured milk protein, (iii) a structured mammalian, avian, or plant protein, or (iv) an unstructured milk protein and a structured mammalian, avian, or plant protein. An illustrative method for preparing a food composition of the disclosure is provided in  FIG.  11   . 
     In some embodiments, the fusion proteins and transgenic plants described herein may be used to prepare a food composition selected from cheese and processed cheese products, yogurt and fermented dairy products, directly acidified counterparts of fermented dairy products, cottage cheese dressing, frozen dairy products, frozen desserts, desserts, baked goods, toppings, icings, fillings, low-fat spreads, dairy-based dry mixes, soups, sauces, salad dressing, geriatric nutrition, creams and creamers, analog dairy products, follow-up formula, baby formula, infant formula, milk, dairy beverages, acid dairy drinks, smoothies, milk tea, butter, margarine, butter alternatives, growing up milks, low-lactose products and beverages, medical and clinical nutrition products, protein/nutrition bar applications, sports beverages, confections, meat products, analog meat products, meal replacement beverages, and weight management food and beverages. 
     In some embodiments the fusion proteins and transgenic plants described herein may be used to prepare a dairy product. In some embodiments, the dairy product is a fermented dairy product. An illustrative list of fermented dairy products includes cultured buttermilk, sour cream, yogurt, skyr, leben, lassi, or kefir. In some embodiments the fusion proteins and transgenic plants described herein may be used to prepare cheese products. 
     In some embodiments the fusion proteins and transgenic plants described herein may be used to prepare a powder containing a milk protein. In some embodiments, the fusion proteins and transgenic plants described herein may be used to prepare a low-lactose product. 
     In some embodiments, a method for making a food composition comprises, expressing a recombinant fusion protein of the disclosure in a plant, extracting the recombinant fusion protein from the plant, optionally separating the milk protein from the structured mammalian or plant protein, and creating a food composition using the fusion protein and/or the milk protein. 
     The recombinant fusion proteins may be extracted from a plant using standard methods known in the art. For example, the fusion proteins may be extracted using solvent or aqueous extraction. In some embodiments, the fusion proteins may be extracted using phenol extraction. Once extracted, the fusion proteins may be maintained in a buffered environment (e.g., Tris, MOPS, HEPES), in order to avoid sudden changes in the pH. The fusion proteins may also be maintained at a particular temperature, such as 4° C. In some embodiments, one or more additives may be used to aid the extraction process (e.g., salts, protease/peptidase inhibitors, osmolytes, reducing agents, etc.) 
     In some embodiments, a method for making a food composition comprises, expressing a recombinant fusion protein of the disclosure in a plant, extracting one or both of the unstructured milk protein and the structured mammalian or plant protein from the plant, and creating a food composition using the milk protein. 
     In some embodiments, the milk protein and the structured mammalian or plant protein are separated from one another in the plant cell, prior to extraction. In some embodiments, the milk protein is separated from the structured mammalian or plant protein after extraction, for example by contacting the fusion protein with an enzyme that cleaves the fusion protein. The enzyme may be, for example, chymosin. In some embodiments, the fusion protein is cleaved using rennet. 
     All references, articles, publications, patents, patent publications, and patent applications cited herein are incorporated by reference in their entireties for all purposes. However, mention of any reference, article, publication, patent, patent publication, and patent application cited herein is not, and should not be taken as an acknowledgment or any form of suggestion that they constitute valid prior art or form part of the common general knowledge in any country in the world, or that they disclose essential matter. 
     EXAMPLES 
     The following experiments demonstrate different recombinant fusion constructs of milk proteins and structured proteins, as well as methods of testing and producing the recombinant proteins, and food compositions produced from the extracted protein. While the examples below describe expression in soybean, it will be understood by those skilled in the art that the constructs and methods disclosed herein may be tailored for expression in any organism. 
     Example 1: Construction of Expression Vectors for Plant Transformation for Stable Expression of Recombinant Fusion Proteins 
     Binary Vector Design 
     While a number of vectors may be utilized for expression of the fusion proteins disclosed herein, the example constructs described below were built in the binary pCAMBIA3300 (Creative Biogene, VET1372) vector, which was customized for soybean transformation and selection. In order to modify the vector, pCAMBIA3300 was digested with HindIII and AseI allowing the release of the vector backbone (LB T-DNA repeat_KanR_pBR322 ori_pBR322 bom_pVS1 oriV_pVs1 repA_pVS1 StaA_RB T-DNA repeat). The 6598 bp vector backbone was gel extracted and a synthesized multiple cloning site (MCS) was ligated via In-Fusion cloning (In-Fusion® HD Cloning System CE, available on the world wide web at clontech.com) to allow modular vector modifications. A cassette containing the  Arabidopsis thaliana  Csr1.2 gene for acetolactate synthase was added to the vector backbone to be used as a marker for herbicide selection of transgenic plants. In order to build this cassette, the regulatory sequences from  Solanum tuberosum  ubiquitin/ribosomal fusion protein promoter (StUbi3 prom; −1 to −922 bp) and terminator (StUbi3 term; 414 bp) (GenBank accession no. L22576.1) were fused to the mutant (S653N) acetolactate synthase gene (Csr1.2; GenBank accession no. X51514.1) (Sathasivan et al, 1990; Ding et al, 2006) to generate imazapyr-resistant traits in soybean plants. The selectable marker cassette was introduced into the digested (EcoRI) modified vector backbone via In-Fusion cloning to form vector pAR15-00 ( FIG.  2   ). 
     Recombinant DNA constructs were designed to express milk proteins (intrinsically unstructured and structured) in transgenic plants. The coding regions of the expression cassettes outlined below contain a fusion of codon-optimized nucleic acid sequences encoding bovine milk proteins, or a functional fragment thereof. To enhance protein expression in soybean, the nucleic acid sequences encoding β-lactoglobulin (GenBank accession no. X14712.1) κ-casein (GenBank accession no. CAA25231), β-casein (GenBank accession no. M15132.1), and αS1-casein (GenBank accession no. X59836.1) were codon optimized using  Glycine max  codon bias and synthesized (available on the world wide web at idtdna.com/CodonOpt). The signal sequences were removed (i.e., making the constructs “truncated”) and the new versions of the genes were renamed as OLG1 (β-lactoglobulin version 1, SEQ ID NO: 9), OLG2 (β-lactoglobulin version 2, SEQ ID NO: 11), OLG3 (β-lactoglobulin version 3, SEQ ID NO: 12), OLG4 (β-lactoglobulin version 4, SEQ ID NO: 13), OKC1-T (Optimized κ-casein Truncated version 1, SEQ ID NO: 3), paraOKC1-T (only the para-κ portion of OKC1-T, SEQ ID NO: 1), OBC-T2 (Optimized β-casein Truncated version 2, SEQ ID NO: 5), and OaS1-T (Optimized αS1-casein Truncated version 1, SEQ ID NO: 7). As will be understood by those skilled in the art, any codon optimized nucleic acid sequences can present from 60% to 100% identity to the native version of the nucleic acid sequence. 
     All the expression cassettes described below and shown in  FIG.  3 - 8    contained codon-optimized nucleic acid sequences encoding bovine milk proteins, or a functional fragment thereof, a seed specific promoter, a 5′UTR, a signal sequence (Sig) that directs foreign proteins to the protein storage vacuoles, and a termination sequence. In some versions of the constructs a linker (FM) such as chymosin cleavage site, was placed between the two proteins and/or a C-terminal KDEL sequence for ER retention was included. Expression cassettes were inserted in the pAR15-00 vector described above utilizing a KpnI restriction site with the MCS ( FIG.  2   ). Coding regions and regulatory sequences are indicated as blocks (not to scale) in  FIG.  3 - 8   . 
     κ-Casein-β-Lactoglobulin Fusion with KDEL 
     Shown in  FIG.  3    is an example expression cassette comprising κ-casein (OKC1-T, SEQ ID NO: 3) and β-lactoglobulin (OLG1, SEQ ID NO: 9). The regulatory sequences that were used in order to produce the heterologous milk proteins in soybean seeds include the promoter of the beta-phaseolin storage protein gene (PvPhas prom; −1 to −1543; GenBank accession no. J01263.1, SEQ ID NO: 18); the 5′UTR of the arc5-1 gene (arc5′UTR; −1 to −13; GenBank accession no. Z50202, SEQ ID NO: 20) (De Jaeger et al, 2002); the signal peptide of Lectin 1 gene 1 (sig10; +1 to +93; GenBank accession no. Glyma.02G012600, SEQ ID NO: 14) (Darnowski et al, 20020); and, the 3′UTR of the arc5-1 gene, (arc term 1197 bp; GenBank accession no. Z50202.1, SEQ ID NO: 21) (De Jaeger et al, 2002). A C-terminal KDEL (SEQ ID NO: 23) was also included for ER retention. 
     β-Casein-β-Lactoglobulin Fusion with Linker 
     Shown in  FIG.  4    is an example expression cassette comprising β-casein (OBC-T2, SEQ ID NO: 5) and β-lactoglobulin (OLG1, SEQ ID NO: 9). The regulatory sequences that were used in order to produce the heterologous milk proteins in soybean seeds include the promoter of the beta-phaseolin storage protein gene (PvPhas prom; −1 to −1543; GenBank accession no. J01263.1, SEQ ID NO: 18); the 5′UTR of the arc5-1 gene (arc5′UTR; −1 to −13; GenBank accession no. Z50202, SEQ ID NO: 20) (De Jaeger et al, 2002); the signal peptide of Lectin 1 gene 1 (sig10; +1 to +93; accession no. Glyma.02G012600, SEQ ID NO: 14) (Darnowski et al, 2002); and, the 3′UTR of the arc5-1 gene, (arc term 1197 bp; accession no. Z50202.1, SEQ ID NO: 21) (De Jaeger, et al 2002). A linker (FM) comprising a chymosin cleavage site was inserted between the two proteins. 
     αS1-Casein-β-Lactoglobulin Fusion with Linker 
     Shown in  FIG.  5    is an example expression cassette comprising αS1-casein (OaS1-T, SEQ ID NO: 7) and β-lactoglobulin (OLG1, SEQ ID NO: 9). The regulatory sequences that were used in order to produce the heterologous milk proteins in soybean seeds include the promoter of the beta-phaseolin storage protein gene (PvPhas prom; −1 to −1543; GenBank accession no. J01263.1, SEQ ID NO: 18); the 5′UTR of the arc5-1 gene (arc5′UTR; -1 to -13; GenBank accession no. Z50202, SEQ ID NO: 20) (De Jaeger et al, 2002); the signal peptide of Lectin 1 gene 1 (sig10; +1 to +93; accession no. Glyma.02G012600, SEQ ID NO: 14) (Darnowski et al, 2002); and, the 3′UTR of the arc5-1 gene, (arc term 1197 bp; GenBank accession no. Z50202.1, SEQ ID NO: 21) (De Jaeger et al, 2002). A linker (FM) comprising a chymosin cleavage site was inserted between the two proteins. 
     Para-κ-Casein-β-Lactoglobulin Fusion with Linker and KDEL 
     Shown in  FIG.  6    is an example expression cassette comprising para-κ-casein (paraOKC1-T, SEQ ID NO: 1) and β-lactoglobulin (OLG1, SEQ ID NO: 9). The regulatory sequences that were used in order to produce the heterologous milk proteins in soybean seeds include the promoter of the beta-phaseolin storage protein gene (PvPhas prom; −1 to −1543; GenBank accession no. J01263.1, SEQ ID NO: 18); the 5′UTR of the arc5-1 gene (arc5′UTR; −1 to −13; GenBank accession no. Z50202, SEQ ID NO: 20) (De Jaeger et al, 2002); the signal peptide of Lectin 1 gene 1 (sig10; +1 to +93; GenBank accession no. Glyma.02G012600, SEQ ID NO: 14) (Darnowski et al, 2002); and, the 3′UTR of the arc5-1 gene, (arc term 1197 bp; GenBank accession no. Z50202.1, SEQ ID NO: 21) (De Jaeger et al 2002). A linker (FM) comprising a chymosin cleavage site was inserted between the two proteins and a C-terminal KDEL (SEQ ID NO: 23) was also included for ER retention. 
     Para-κ-Casein-β-Lactoglobulin Fusion with Linker 
     Shown in  FIG.  7    is an example expression cassette comprising para-κ-casein (paraOKC1-T, SEQ ID NO: 1) and β-lactoglobulin (OLG1, SEQ ID NO: 9). The regulatory sequences that were used in order to produce the heterologous milk proteins in soybean seeds include the promoter of the beta-phaseolin storage protein gene (PvPhas prom; −1 to −1543; GenBank accession no. J01263.1, SEQ ID NO: 18); the 5′UTR of the arc5-1 gene (arc5′UTR; -1 to -13; GenBank accession no. Z50202, SEQ ID NO: 20) (De Jaeger et al, 2002); the signal peptide of Lectin 1 gene 1 (sig10; +1 to +93; GenBank accession no. Glyma.02G012600, SEQ ID NO: 14) (Darnowski et al, 2002); and, the 3′UTR of the arc5-1 gene, (arc term 1197 bp; GenBank accession no. Z50202.1, SEQ ID NO: 21) (De Jaeger et al, 2002). A linker (FM) comprising a chymosin cleavage site was inserted between the two proteins. 
     Fusion Protein with Seed2 Promoter, Sig2 and Nopaline Synthase Terminator 
     Shown in  FIG.  8    is an example expression cassette comprising κ-casein (OKC1-T, SEQ ID NO: 3) and β-lactoglobulin (OLG1, SEQ ID NO: 9). The regulatory sequences that were used in order to produce the heterologous milk proteins in soybean seeds include the promoter and signal peptide of glycinin 1 (GmSeed2 (SEQ ID NO: 19):sig2 (SEQ ID NO: 16)) followed by the ER retention signal (KDEL) and the Nopaline synthase termination sequence (nos term, SEQ ID NO: 22). 
     Example 2: Identification of Transgenic Events, Recombinant Protein Extraction and Detection 
     To quantify recombinant protein expression levels, DNA constructs such as those shown in  FIG.  3 - 8    were transformed into soybean using transformation protocols well known in the art, for example, by bombardment or agrobacterium. Total soybean genomic DNA was isolated from the first trifoliate leaves of transgenic events using the PureGene tissue DNA isolation kit (product #158667: QIAGEN, Valencia, Calif., USA). Trifoliates were frozen in liquid nitrogen and pulverized. Cells were lysed using the PureGene Cell Lysis Buffer, proteins were precipitated using the PureGene Protein Precipitation Buffer, and DNA was precipitated from the resulting supernatant using ethanol. The DNA pellets were washed with 70% ethanol and resuspended in water. 
     Genomic DNA was quantified by the Quant-iT PicoGreen (product #P7589: ThermoFisher Scientific, Waltham, Mass., USA) assay as described by manufacturer, and 150 ng of DNA was digested overnight with EcoRI, HindIII, NcoI, and/or KpnI, 30 ng of which was used for a BioRad ddPCR reaction, including labelled FAM or HEX probes for the transgene and Lectinl endogenous gene respectively. Transgene copy number (CNV) was calculated by comparing the measured transgene concentration to the reference gene concentration. A CNV of greater than or equal to one was deemed acceptable. 
     Preparation of Total Soluble Protein Samples 
     Total soluble soybean protein fractions were prepared from the seeds of transgenic events by bead beating seeds (seeds collected about 90 days after germination) at 15000 rpm for 1 min. The resulting powder was resuspended in 50 mM Carbonate-Bicarbonate pH 10.8, 1 mM DTT, 1× HALT Protease Inhibitor Cocktail (Product #78438 ThermoFisher Scientific). The resuspended powder was incubated at 4° C. for 15 minutes and then the supernatant collected after centrifuging twice at 4000 g, 20 min, 4° C. Protein concentration was measured using a modified Bradford assay (Thermo Scientific Pierce 660 nm assay; Product #22660 ThermoFisher Scientific) using a bovine serum albumin (BSA) standard curve. 
     Recombinant Protein Quantification via Western Blot Densitometry 
     SDS-PAGE was performed according to manufacturer&#39;s instructions (Product#5678105BioRad, Hercules, CA, USA) under denaturing and reducing conditions. 5 ug of total protein extracts were loaded per lane. For immunoblotting proteins separated by SD S-PAGE were transferred to a PVDF membrane using Trans-Blot® Turbo™ Midi PVDF Transfer Packs (Product#1704157 BioRad) according to manufacturer&#39;s guidelines. Membranes were blocked with 3% BSA in phosphate buffered saline with 0.5% Tween-20, reacted with antigen specific antibody and subsequently reacted with fluorescent goat anti rabbit IgG (Product #60871 BioRad, CA). Membranes were scanned according to manufacturer&#39;s instructions using the ChemiDoc MP Imaging System (BioRad, CA) and analyzed using ImageLab Version 6.0.1 Standard Edition (Bio-Rad Laboratories, Inc.). Recombinant protein from the seeds of transgenic events was quantified by densitometry from commercial reference protein spike-in standards. 
     Shown in  FIGS.  9 A,  9 B,  9 C, and  9 D  are Western Blots of protein extracted from transgenic soybeans expressing the κ-casein-β-lactoglobulin expression cassette shown in  FIG.  3   .  FIG.  9 A  shows the fusion protein detected using a primary antibody raised against κ-casein. The first lane is a molecular weight marker. Lanes two (DCI 9.1) and three (DCI 9.2) represent individual seeds from a single transgenic line. Lane four (DCI 3.1) represents a seed from a separate transgenic line. Lane five is protein extracted from wild-type soybean plants, and lanes six-eight are protein extracted from wild-type soybean plants spiked with 0.05% commercial κ-casein (lane 6), 0.5% commercial κ-casein (lane 7), and 1.5% commercial κ-casein (lane 8). The κ-casein commercial protein is detected at an apparent molecular weight (MW) of ˜26 kDa (theoretical: 19 kDa-arrow). The fusion protein is detected at an apparent MW of ˜40 kDa (theoretical: 38 kDa-arrowhead). 
       FIG.  9 B  shows the fusion protein detected using a primary antibody raised against β-lactoglobulin. The first lane is a molecular weight marker. Lanes two (DCI 9.1) and three (DCI 9.2) represent individual seeds from a single transgenic line. Lane four (DCI 3.1) represents a seed from a separate transgenic line. Lane five is protein extracted from wild-type soybean plants, and lanes six-eight are protein extracted from wild-type soybean plants spiked with 0.05% commercial β-lactoglobulin (lane 6), 1% commercial β-lactoglobulin (lane 7), and 2% commercial β-lactoglobulin (lane 8). The β-lactoglobulin commercial protein is detected at an apparent MW of ˜18 kDa (theoretical: 18 kDa-arrow). The fusion protein is detected at an apparent MW of ˜40 kDa (theoretical: 38 kDa-arrowhead).  FIGS.  9 C and  9 D  show the protein gels as control for equal lane loading (image is taken at the end of the SDS run) for  FIGS.  9 A and  9 B , respectively. 
     Other combinations of structured and unstructured proteins were tested and evaluated for the percentage of recombinant protein. Cassettes having the same promoter (Seed2-sig), signal peptide (EUT:Rb7T), and in some instances a different terminator, were built with either α-S1-casein, β-casein, κ-casein, or the fusion of β-lactoglobulin with κ-casein (kCN-LG) (See  FIGS.  3  and  8   ). As shown below in Table 3, none of the cassettes encoding α-S1-casein, β-casein, or κ-casein were able to produce expression of the protein at a level that exceeded 1% total soluble protein. However, when κ-casein was fused with β-lactoglobulin, κ-casein was expressed at a level that was greater than 1% total soluble protein. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Expression levels of unstructured proteins 
               
            
           
           
               
               
               
            
               
                   
                   
                 Number of events 1   
               
               
                   
                   
                 accumulating the recombinant 
               
               
                   
                 Total 
                 protein at the concentration: 
               
            
           
           
               
               
               
               
            
               
                   
                 events 1   
                 0-1% 
                 Above 1% 
               
               
                   
                 analyzed 
                 TSP 
                 TSP 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Unstructured 
                 κ-Casein 
                 89 
                 89 
                 0 
               
               
                   
                 B-Casein 
                 12 
                 12 
                 0 
               
               
                   
                 αS1-Casein 
                 6 
                 6 
                 0 
               
               
                 Fusion 
                 kCN-LG 
                 23 
                 12 
                 11 
               
               
                   
               
               
                   1 As used in Table 3, the each “event” refers to an independent transgenic line. 
               
            
           
         
       
     
     As will be readily understood by those of skill in the art, T-DNA insertion into the plant genome is a random process and each T-DNA lands at an unpredictable genomic position. Hence, each of the 23 events generated in Table 3 for the fusion protein have different genomic insertion loci. The genomic context greatly influences the expression levels of a gene, and each loci will be either favorable or unfavorable for the expression of the recombinant genes. The variability observed at the protein level is a reflection of that random insertion process, and explains why 12 out of 23 events present expression levels below 1%. 
     Example 3: Food Compositions 
     The transgenic plants expressing the recombinant fusion proteins described herein can produce milk proteins for the purpose of food industrial, non-food industrial, pharmaceutical, and commercial uses described in this disclosure. An illustrative method for making a food composition is provided in  FIG.  11   . 
     A fusion protein comprising an unstructured milk protein (para-κ-casein) and a structured mammalian protein (β-lactoglobulin) is expressed in a transgenic soybean plant. The fusion protein comprises a chymosin cleavage site between the para-κ-casein and the β-lactoglobulin. 
     The fusion protein is extracted from the plant. The fusion protein is then treated with chymosin, to separate the para-κ-casein from the β-lactoglobulin. The para-κ-casein is isolated and/or purified and used to make a food composition (e.g., cheese). 
     NUMBERED EMBODIMENTS 
     Notwithstanding the appended claims, the following numbered embodiments also form part of the instant disclosure. 
     1. A stably transformed plant comprising in its genome: a recombinant DNA construct encoding a fusion protein, the fusion protein comprising: (i) an unstructured milk protein, and (ii) a structured animal protein; wherein the fusion protein is stably expressed in the plant in an amount of 1% or higher per total protein weight of soluble protein extractable from the plant. 
     2. The stably transformed plant of embodiment 1, wherein the fusion protein comprises, from N-terminus to C-terminus, the unstructured milk protein and the animal protein. 
     3. The stably transformed plant of any one of embodiments 1-2, wherein the unstructured milk protein is α-S1 casein, α-S2 casein, β-casein, or κ-casein. 
     4. The stably transformed plant of embodiment 1, wherein the unstructured milk protein is κ-casein and comprises the sequence of SEQ ID NO: 4, or a sequence at least 90% identical thereto. 
     5. The stably transformed plant of embodiment 1, wherein the unstructured milk protein is para-κ-casein and comprises the sequence of SEQ ID NO: 2, or a sequence at least 90% identical thereto. 
     6. The stably transformed plant of embodiment 1, wherein the unstructured milk protein is β-casein and comprises the sequence of SEQ ID NO: 6, or a sequence at least 90% identical thereto. 
     7. The stably transformed plant of embodiment 1, wherein the unstructured milk protein is α-S1 casein and comprises the sequence SEQ ID NO: 8, or a sequence at least 90% identical thereto. 
     8. The stably transformed plant of embodiment 1, wherein the unstructured milk protein is α-S2 casein and comprises the sequence SEQ ID NO: 84, or a sequence at least 90% identical thereto. 
     9. The stably transformed plant of any one of embodiments 1-8, wherein the structured animal protein is a structured mammalian protein. 
     10. The stably transformed plant of embodiment 9, wherein the structured mammalian protein is β-lactoglobulin, α-lactalbumin, albumin, lysozyme, lactoferrin, lactoperoxidase, hemoglobin, collagen, or an immunoglobulin. 
     11. The stably transformed plant of embodiment 9, wherein the structured mammalian protein is β-lactoglobulin and comprises the sequence of SEQ ID NO: 10, or a sequence at least 90% identical thereto. 
     12. The stably transformed plant of any one of embodiments 1-8, wherein the structured animal protein is a structured avian protein. 
     13. The stably transformed plant embodiment 12, wherein the structured avian protein is ovalbumin, ovotransferrin, lysozyme or ovoglobulin. 
     14. The stably transformed plant of embodiment 9, wherein the milk protein is κ-casein and the structured mammalian protein is β-lactoglobulin. 
     15. The stably transformed plant of embodiment 9, wherein the milk protein is para-κ-casein and the structured mammalian protein is β-lactoglobulin. 
     16. The stably transformed plant of embodiment 9, wherein the milk protein is β-casein and the structured mammalian protein is β-lactoglobulin. 
     17. The stably transformed plant of embodiment 9, wherein the milk protein is α-S1 casein or α-S2 casein and the structured mammalian protein is β-lactoglobulin. 
     18. The stably transformed plant of any one of embodiments 1-17, wherein the plant is a dicot. 
     19. The stably transformed plant of embodiment 18, wherein the dicot is  Arabidopsis , tobacco, tomato, potato, sweet potato, cassava, alfalfa, lima bean, pea, chick pea, soybean, carrot, strawberry, lettuce, oak, maple, walnut, rose, mint, squash, daisy, Quinoa, buckwheat, mung bean, cow pea, lentil, lupin, peanut, fava bean, French beans (i.e., common beans), mustard, or cactus. 
     20. The stably transformed plant of any one of embodiments 1-19, wherein the plant is soybean. 
     21. The stably transformed plant of any one of embodiments 1-20, wherein the recombinant DNA construct is codon-optimized for expression in the plant. 
     22. The stably transformed plant of any one of embodiments 1-21, wherein the fusion protein comprises a protease cleavage site. 
     23. The stably transformed plant of embodiment 22, wherein the protease cleavage site is a chymosin cleavage site. 
     24. The stably transformed plant of any one of embodiments 1-23, wherein the fusion protein is expressed at a level at least 2-fold higher than an unstructured milk protein expressed individually in a plant. 
     25. The stably transformed plant of any one of embodiments 1-24, wherein the fusion protein accumulates in the plant at least 2-fold higher than an unstructured milk protein expressed without the structured animal protein. 
     26. A recombinant fusion protein comprising: (i) an unstructured milk protein, and (ii) a structured animal protein. 
     27. The recombinant fusion protein of embodiment 26, wherein the fusion protein is expressed in a plant. 
     28. The recombinant fusion protein of embodiment 26 or 27, wherein the unstructured milk protein is α-S1 casein, α-S2 casein, β-casein, or κ-casein. 
     29. The recombinant fusion protein of embodiment 28, wherein the milk protein is κ-casein and comprises the sequence of SEQ ID NO: 4, or a sequence at least 90% identical thereto. 
     30. The recombinant fusion protein of embodiment 28, wherein the milk protein is para-κ-casein and comprises the sequence of SEQ ID NO: 2, or a sequence at least 90% identical thereto. 
     31. The recombinant fusion protein of embodiment 28, wherein the milk protein is β-casein and comprises the sequence of SEQ ID NO: 6, or a sequence at least 90% identical thereto. 
     32. The recombinant fusion protein of embodiment 28, wherein the milk protein is α-S1 casein and comprises the sequence SEQ ID NO: 8, or a sequence at least 90% identical thereto. 
     33. The recombinant fusion protein of embodiment 28, wherein the milk protein is α-S2 casein and comprises the sequence SEQ ID NO: 84, or a sequence at least 90% identical thereto. 
     34. The recombinant fusion protein of any one of embodiments 26-33, wherein the structured animal protein is a structured mammalian protein. 
     35. The recombinant fusion protein of embodiment 34, wherein the structured mammalian protein is β-lactoglobulin, α-lactalbumin, albumin, lysozyme, lactoferrin, lactoperoxidase, hemoglobin, collagen, or an immunoglobulin. 
     36. The recombinant fusion protein of embodiment 34, wherein the structured mammalian protein is β-lactoglobulin and comprises the sequence of SEQ ID NO: 10, or a sequence at least 90% identical thereto. 
     37. The recombinant fusion protein of any one of embodiments 26-33, wherein the structured animal protein is a structured avian protein. 
     38. The recombinant fusion protein of embodiment 37, wherein the structured avian protein is ovalbumin, ovotransferrin, lysozyme or ovoglobulin. 
     39. The recombinant fusion protein embodiment 34, wherein the milk protein is κ-casein and the structured mammalian protein is β-lactoglobulin. 
     40. The recombinant fusion protein of embodiment 34, wherein the milk protein is para-κ-casein and the structured mammalian protein is β-lactoglobulin. 
     41. The recombinant fusion protein of embodiment 34, wherein the milk protein is β-casein and the structured mammalian protein is β-lactoglobulin. 
     42. The recombinant fusion protein of embodiment 34, wherein the milk protein is α-S1 casein or α-S2 casein and the structured mammalian protein is β-lactoglobulin. 
     43. The recombinant fusion protein of embodiment 34, wherein the fusion protein comprises a protease cleavage site. 
     44. The recombinant fusion protein of embodiment 34, wherein the protease cleavage site is a chymosin cleavage site. 
     45. A nucleic acid encoding the recombinant fusion protein of any one of embodiments 26 to 44. 
     46. The nucleic acid of embodiment 45, wherein the nucleic acid is codon optimized for expression in a plant species. 
     47. The nucleic of embodiment 45 or 46, wherein the nucleic acid is codon optimized for expression in soybean. 
     48. A vector comprising a nucleic acid encoding a recombinant fusion protein, wherein the recombinant fusion protein comprises: (i) an unstructured milk protein, and (ii) a structured animal protein. 
     49. The vector of embodiment 48, wherein the vector is a plasmid. 
     50. The vector of embodiment 49, wherein the vector is an  Agrobacterium  Ti plasmid. 
     51. The vector of any one of embodiments 48-50, wherein the nucleic acid comprises, in order from 5′ to 3′: a promoter; a 5′ untranslated region; a sequence encoding the fusion protein; and a terminator. 
     52. The vector of embodiment 51, wherein the promoter is a seed-specific promoter. 
     53. The vector of embodiment 52, wherein the seed-specific promoter is selected from the group consisting of PvPhas, BnNap, AtOle1, GmSeed2, GmSeed3, GmSeed5, GmSeed6, GmSeed7, GmSeed8, GmSeed10, GmSeed11, GmSeed12, pBCON, GmCEP1-L, GmTHIC, GmBg7S1, GmGRD, GmOLEA, GmOLER, Gm2S-1, and GmBBld-II. 
     54. The vector of embodiment 53, wherein the seed-specific promoter is PvPhas and comprises the sequence of SEQ ID NO: 18, or a sequence at least 90% identical thereto. 
     55. The vector of embodiment 53, wherein the seed-specific promoter is GmSeed2 and comprises the sequence of SEQ ID NO: 19, or a sequence at least 90% identical thereto. 
     56. The vector of any one of embodiments 51-55, wherein the 5′ untranslated region is selected from the group consisting of Arc5′UTR and glnB1UTR. 
     57. The vector of embodiment 56, wherein the 5′ untranslated region is Arc5′UTR and comprises the sequence of SEQ ID NO: 20, or a sequence at least 90% identical thereto. 
     58. The vector of any one of embodiments 51-57, wherein the expression cassette comprises a 3′ untranslated region. 
     59. The vector of embodiment 58, wherein the 3′ untranslated region is Arc5-1 and comprises SEQ ID NO: 21, or a sequence at least 90% identical thereto. 
     60. The vector of any one of embodiments 51-59, wherein the terminator sequence is a terminator isolated or derived from a gene encoding Nopaline synthase, Arc5-1, an Extensin, Rb7 matrix attachment region, a Heat shock protein, Ubiquitin 10, Ubiquitin 3, and M6 matrix attachment region. 
     61. The vector of embodiment 60, wherein the terminator sequence is isolated or derived from a  Nopaline synthase  gene and comprises the sequence of SEQ ID NO: 22, or a sequence at least 90% identical thereto. 
     62. A plant comprising the recombinant fusion protein of any one of embodiments 26-44 or the nucleic acid of any one of embodiments 45-47. 
     63. A method for stably expressing a recombinant fusion protein in a plant, the method comprising: a) transforming a plant with a plant transformation vector comprising an expression cassette comprising: a sequence encoding a fusion protein, wherein the fusion protein comprises an unstructured milk protein, and a structured animal protein; and b) growing the transformed plant under conditions wherein the recombinant fusion protein is expressed in an amount of 1% or higher per total protein weight of soluble protein extractable from the plant. 
     64. The method of embodiment 63, wherein the unstructured milk protein is κ-casein. 
     65. The method of embodiment 63 or 64, wherein the structured animal protein is β-lactoglobulin. 
     66. A food composition comprising the recombinant fusion protein of any one of embodiments 26-44. 
     67. A method for making a food composition, the method comprising: expressing the recombinant fusion protein of any one of embodiments 26-44 in a plant; extracting the recombinant fusion protein from the plant; optionally, separating the milk protein from the structured animal protein or the structured plant protein; and creating a food composition using the milk protein or the fusion protein. 
     68. The method of embodiment 67, wherein the plant stably expresses the recombinant fusion protein. 
     69. The method of embodiment 68, wherein the plant expresses the recombinant fusion protein in an amount of 1% or higher per total protein weight of soluble protein extractable from the plant. 
     70. The method of any one of embodiments 67-69, wherein the plant is soybean. 
     71. The method of any one of embodiments 67-70, wherein the food composition comprises the structured animal or plant protein. 
     72. The method of any one of embodiments 67-71, wherein the milk protein and the structured animal or plant protein are separated from one another in the plant cell, prior to extraction. 
     73. The method of any one of embodiments 67-71, wherein the milk protein is separated from the structured animal or plant protein after extraction, by contacting the fusion protein with an enzyme that cleaves the fusion protein. 
     74. A food composition produced using the method of any one of embodiments 67-73. 
     75. A plant-expressed recombinant fusion protein, comprising: κ-casein; and β-lactoglobulin. 
     76. The plant-expressed recombinant fusion protein of embodiment 75, wherein the fusion protein comprises, in order from N-terminus to C-terminus, the κ-casein and the β-lactoglobulin. 
     77. The plant-expressed recombinant fusion protein of embodiment 75 or 76, wherein the fusion protein comprises a protease cleavage site. 
     78. The plant-expressed recombinant fusion protein of embodiment 77, wherein the protease cleavage site is a chymosin cleavage site. 
     79. The plant-expressed recombinant fusion protein of any one of embodiments 75-78, wherein the fusion protein comprises a signal peptide. 
     80. The plant-expressed recombinant fusion protein of embodiment 79, wherein the signal peptide is located at the N-terminus of the fusion protein. 
     81. The plant-expressed recombinant fusion protein of any one of embodiments 75-80, wherein the fusion protein is encoded by a nucleic acid that is codon optimized for expression in a plant. 
     82. The plant-expressed recombinant fusion protein of any one of embodiments 75-81, wherein the fusion protein is expressed in a soybean. 
     83. The plant-expressed recombinant fusion protein of any one of embodiments 75-81, wherein the fusion protein has a molecular weight of 30 kDa to 50 kDa. 
     84. The plant-expressed recombinant fusion protein of any one of embodiments 75-83, wherein the fusion protein is expressed in a plant in an amount of 1% or higher per total protein weight of soluble protein extractable from the plant. 
     85. The plant-expressed recombinant fusion protein of any one of embodiments 75-84, wherein the fusion protein is expressed in the plant at a level at least 2-fold higher than κ-casein expressed individually in a plant. 
     86. The plant-expressed recombinant fusion protein of any one of embodiments 75-84, wherein the fusion protein accumulates in the plant at least 2-fold higher than κ-casein expressed without β-lactoglobulin. 
     87. A stably transformed plant, comprising in its genome: a recombinant DNA construct encoding a fusion protein, the fusion protein comprising: κ-casein; and β-lactoglobulin; wherein the fusion protein is stably expressed in the plant in an amount of 1% or higher per total protein weight of soluble protein extractable from the plant. 
     88. The stably transformed plant of embodiment 87, wherein the fusion protein comprises, in order from N-terminus to C-terminus, the κ-casein and the β-lactoglobulin. 
     89. The stably transformed plant of embodiment 87 or 88, wherein the fusion protein comprises a protease cleavage site. 
     90. The stably transformed plant of embodiment 89, wherein the protease cleavage site is a chymosin cleavage site. 
     91. The stably transformed plant of any one of embodiments 87-90, wherein the fusion protein comprises a signal peptide. 
     92. The stably transformed plant of embodiment 91, wherein the signal peptide is located at the N-terminus of the fusion protein. 
     93. The stably transformed plant of any one of embodiments 87-92, wherein the plant is soybean. 
     94. The stably transformed plant of any one of embodiments 87-93, wherein the recombinant DNA construct comprises codon-optimized nucleic acids for expression in the plant. 
     95. The stably transformed plant of any one of embodiments 87-94, wherein the fusion protein has a molecular weight of 30 kDa to 50 kDa. 
     96. The stably transformed plant of any one of embodiments 87-95, wherein the fusion protein is expressed at a level at least 2-fold higher than κ-casein expressed individually in a plant. 
     97. The stably transformed plant of any one of embodiments 87-96, wherein the fusion protein accumulates in the plant at least 2-fold higher than κ-casein expressed without β-lactoglobulin. 
     98. A plant-expressed recombinant fusion protein comprising: a casein protein and β-lactoglobulin. 
     99. The plant-expressed recombinant fusion protein of embodiment 98, wherein the casein protein is α-S1 casein, α-S2 casein, β-casein, or κ-casein. 
     100. A stably transformed plant, comprising in its genome: a recombinant DNA construct encoding a fusion protein, the fusion protein comprising: a casein protein and β-lactoglobulin; wherein the fusion protein is stably expressed in the plant in an amount of 1% or higher per total protein weight of soluble protein extractable from the plant. 
     101. The stably transformed plant of embodiment 100, wherein the casein protein is α-S1 casein, α-S2 casein, β-casein, or κ-casein.