A protein structure capable of selective interaction with an organic target selected from the group consisting of the fragment crystallisable (F c) region of IgG and molecules comprising IgG or derivatives thereof, wherein said protein structure is a polymer comprising as a repeating structural unit a recombinant fusion protein that is capable of selective interaction with the organic target and comprising the moieties B, REP and CT, wherein the polymer is comprising more than 100 fusion protein structural units, and wherein: B is a non-spidroin moiety of more than 30 amino acid residues, which provides the capacity of selective interaction with the organic target, wherein the B moiety is selected from the group consisting of the Z domain of staphylococcal protein A, staphylococcal protein A and the E, D, A, B and C domains thereof; and protein fragments having at least 70% identity to any of these amino acid sequences; the REP and CT moieties provide the capacity of forming a polymer; REP is a moiety of from 70 to 300 amino acid residues, selected from the group consisting of L n L, L n AL, L n L, L n GL, wherein n is an integer from 2 to 10; each individual A segment is an amino acid sequence of from 8 to 18 amino acid residues, wherein from 0 to 3 of the amino acid residues are not Ala, and the remaining amino acid residues are Ala; each individual G segment is an amino acid sequence of from 12 to 30 amino acid residues, wherein at least 40% of the amino acid residues are Gly; and each individual L segment is a linker amino acid sequence of from 0 to 20 amino acid residues; and CT is a moiety of from 70 to 120 amino acid residues and has at least 50% identity to SEQ ID NO: 9 or at least 80% identity to SEQ ID NO: 7. B is a non-spidroin moiety of more than 30 amino acid residues, which provides the capacity of selective interaction with the organic target, wherein the B moiety is selected from the group consisting of the Z domain of staphylococcal protein A, staphylococcal protein A and the E, D, A, B and C domains thereof; and protein fragments having at least 70% identity to any of these amino acid sequences; the REP and CT moieties provide the capacity of forming a polymer; REP is a moiety of from 70 to 300 amino acid residues, selected from the group consisting of L n L, L n AL, L n L, L n GL, wherein n is an integer from 2 to 10; each individual A segment is an amino acid sequence of from 8 to 18 amino acid residues, wherein from 0 to 3 of the amino acid residues are not Ala, and the remaining amino acid residues are Ala; each individual G segment is an amino acid sequence of from 12 to 30 amino acid residues, wherein at least 40% of the amino acid residues are Gly; and each individual L segment is a linker amino acid sequence of from 0 to 20 amino acid residues; and n is an integer from 2 to 10; each individual A segment is an amino acid sequence of from 8 to 18 amino acid residues, wherein from 0 to 3 of the amino acid residues are not Ala, and the remaining amino acid residues are Ala; each individual G segment is an amino acid sequence of from 12 to 30 amino acid residues, wherein at least 40% of the amino acid residues are Gly; and each individual L segment is a linker amino acid sequence of from 0 to 20 amino acid residues; and CT is a moiety of from 70 to 120 amino acid residues and has at least 50% identity to SEQ ID NO: 9 or at least 80% identity to SEQ ID NO: 7.