...response: Uncoating the pathway to the vacuole in Arabidopsis

We are in close agreement with Miller and Anderson's article1xUncoating the mechanisms of vacuolar protein transport. Miller, E.A. and Anderson, M.A. Trends Plant Sci. 1998; 4: 46–48Abstract | Full Text | Full Text PDFSee all References1, but would like to add a few extra points for discussion. For example, we note that they have combined the results reported for Arabidopsis with those found in the specialized cells of the barley root tip and pea cotyledon. These latter cells are highly specialized for protein storage and appear to have more than one type of vacuole (see citations in Ref. 1xUncoating the mechanisms of vacuolar protein transport. Miller, E.A. and Anderson, M.A. Trends Plant Sci. 1998; 4: 46–48Abstract | Full Text | Full Text PDFSee all ReferencesRef. 1). Clearly, not all cells in a plant are so specialized and many will contain only the basic pathway(s) for transport to the vacuole. Can we really compare the behavior of the secretory system of cells specialized for trafficking to two or more vacuoles to the more basic pathway found in all cells? Although we are also interested in the adaptations necessary to traffic cargo to two distinct vacuoles, we have focused on examining the basic pathway because it is easier to understand what is ‘special’ if one first understands what is ‘normal’.Arabidopsis has proved to be a useful model system for trafficking studies because of the availability of DNA sequence data and gene-knockout techniques. It has thereby been possible to characterize the putative vacuolar cargo receptor AtELP, the t-SNARE AtPEP12p, the Sec1p-homolog AtVPS45p (see 2xA putative vacuolar cargo receptor partially colocalizes with AtPEP12p on a prevacuolar compartment in Arabidopsis roots. Sanderfoot, A.A. et al. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 9920–9925Crossref | PubMed | Scopus (133)See all References, 3xAn Arabidopsis VPS45p homolog implicated in protein transport to the vacuole. Bassham, D.C. and Raikhel, N.V. Plant Physiol. 1998; 117: 407–415Crossref | PubMedSee all References), and other SNAREs we are currently studying (AtPLP, AtSED5p, AtTLG2p, AtVTI1p). These components appear to be expressed in all cell-types, suggesting that they are part of the basic machinery. Ironically, we now have many potential members of the vacuolar targeting machinery, but little idea about the cargo this machinery transports. Very few endogenous soluble vacuolar proteins have been characterized in Arabidopsis. We, and others, have found that many of the vacuolar proteins or associated enzymatic activities characterized in other systems (such as glucanases and chitinases) appear to have both vacuolar and secreted forms in Arabidopsis, and this dual localization has greatly hampered their use as endogenous markers. Nevertheless, vacuolar markers that are not cell- or tissue-specific (i.e. storage proteins) are rare in many systems.In general, we have found this basic pathway to be similar to that found in yeast and mammalian cells, but it is premature to suggest that plants follow all the usual rules. It is possible that cells with only a single vacuole could have several independent targeting pathways4xDifferent sensitivities to wortmannin of two vacuolar sorting signals indicated the presence of distinct sorting machineries in tobacco cells. Matsuoka, K. et al. J. Cell Biol. 1995; 130: 1307–1318Crossref | PubMed | Scopus (189)See all References4. In addition, our examination of the Arabidopsis SNARE machinery has revealed multiple isoforms of many of the v- and t-SNAREs (including AtPEP12p) that are found as single copy genes in yeast. This result comes at a time when the Arabidopsis genome sequencing project is far from complete: with so much of the genome left to sequence, the number of SNAREs in Arabidopsis is likely to greatly exceed that found in yeast. Is this a sign of SNARE redundancy, or is it further evidence that the plant secretory system is highly complex?With regard to the combined model presented by Miller and Anderson1xUncoating the mechanisms of vacuolar protein transport. Miller, E.A. and Anderson, M.A. Trends Plant Sci. 1998; 4: 46–48Abstract | Full Text | Full Text PDFSee all References1, we think it important to add some points. Even if we assume that AtELP/BP-80 is involved in selection and packaging of cargo with N-terminal vacuolar sorting signals (VSS) into clathrin-coated vesicles, and that AtPEP12p is the receptor on the prevacuolar compartment (PVC) for these vesicles (and we have yet to show this conclusively), much remains unknown. Many proteins enter the vacuole using signals other than an N-terminal VSS. Vacuolar cargo with C-terminal VSS arrive at the same vacuole, and do so even in cells that do not appear to have dense vesicles5xColocalization of barley lectin and sporamin in vacuoles of transgenic tobacco plants. Schroeder, M.R. et al. Plant Physiol. 1993; 101: 451–458Crossref | PubMedSee all References5. Perhaps the hypothesis that the type of VSS determines the type of vesicle1xUncoating the mechanisms of vacuolar protein transport. Miller, E.A. and Anderson, M.A. Trends Plant Sci. 1998; 4: 46–48Abstract | Full Text | Full Text PDFSee all References1 is too simplistic. If this C-terminal VSS cargo travels in a vesicle distinct from clathrin coated vesicles and dense vesicles, then how is this cargo selected and packaged into these vesicles? Do the multiple forms of AtELP/BP-80 that appear to exist in Arabidopsis work in the same pathway, or do some function with these other VSS? Is AtPEP12p the receptor at the PVC for all of these different vesicles, or could this be the role for the AtPEP12p-isoforms? Do all of these vesicles pass through the same or different PVCs? Could some of these vesicles even bypass the PVC and go directly to the vacuole? An understanding of the basic pathway could help clear up which of these questions correspond to the overall complexity of the plant system and which are cell-specific modifications.We have highlighted just some of the discussion concerning the secretory system of plant cells. To continue this process, we would like to draw the readers' attention to an electronic mailing list specifically for researchers interested in the secretory system of plants (secpatwy@list.msu.edu). We would encourage anyone willing to share ideas (or useful vacuolar markers from Arabidopsis) to subscribe and participate.