Seed PSVs and Protein Sorting

Seeds that are rich in proteins usually contain numerous PSVs (Fig. 29.1a). These PSVs are surrounded by a single membrane and can be defined by the presence of abundant levels of tonoplast intrinsic protein (TIP) in their tonoplasts (Fig. 29.1b). When viewed under a transmission electron microscope (TEM), PSVs in most seeds, including tobacco (Nicotiana tabacum L.), contain three morphologically and functionally distinct sub-compartments (Fig. 29.1c and d): crystalloid, matrix and globoid (Jiang et al., 2000, 2001, 2002). The matrix and crystalloid contain storage proteins, while the globoid contains salts of phytic acid. However, recent studies have indicated that the globoid, which is surrounded by a single unique membrane, might represent a lytic compartment within the PSV because it contains marker proteins for the LV pathway (Jiang et al., 2001).

Fig. 29.1. The structure of protein storage vacuoles (PSVs) in mature tobacco seeds. Paraffin-embedded sections were prepared from mature seeds. The top panels demonstrate PSVs in cells visualized by differential interference contrast (a) and by staining with anti-a-TIP visualized with epifluorescence (b). Panel c shows a TEM image of a single PSV that contains three distinct sub-compartments, which is also represented diagrammatically (d). M = matrix; C = crystalloid; G = globoid. Scale bar = 50 |im in a and b and 2 |im in c.

Multiple mechanisms are responsible for transporting storage proteins to PSVs. In rice (Oryza sativa L.), storage proteins reach protein bodies either through the Golgi apparatus or directly from the endoplasmic reticulum (ER) (Okita and Rogers, 1996). Aggregation of storage proteins has also been suggested to play a role in transporting the storage protein 7S vicillin to PSVs in pea (Pisumsativum L.) (Robinson et al., 1998). Recently, a member of the Arabidopsis (Arabidopsis thaliana (L.) Heynh.) VSR group of proteins and the Arabidopsis receptor-like protein RMR (Jiang et al., 2000), have been shown to function as receptors in the transport of the storage proteins globulin and phaseolin, respectively, to PSVs in Arabidopsis (Shimada et al., 2003; Park et al., 2005).

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