The Golgi bypassed

Plants have two, perhaps three, types of vacuoles with different functions, sometimes within a single cell. Each type is characterized by a different aquaporin, alpha, gamma or delta TIP (tonoplast intrinsic protein). These recently discovered extra destinations make targeting within the secretory system in plants much more complex than previously thought. This paper investigates the targeting determinants of two integral membrane proteins to find how proteins are delivered to their specific destination. The results suggest that plants have a route from the endoplasmic reticulum to the vacuole that is independent of the Golgi apparatus and suggest that transmembrane domains (TMDs) and cytoplasmic C-terminal tails are involved in this targeting.The lysosome-like lytic vacuole (LV) is characterized by gamma-TIP, low pH and soluble proteases such as the aleurain. A putative sorting receptor, BP-80, is believed to mediate traffic of soluble proteins from the Golgi towards this vacuole. BP-80 consists of a large, lumenal N-terminal domain, a single TMD and a short C-terminal domain (CTD) with a motif that can bind to adaptins, consistent with the idea that clathrin is involved in its trafficking. Alpha-TIP on the other hand is associated with protein storage vacuoles (PSVs). It has six membrane spans and a short cytosolic C-terminus that is rich in proline and histidine. Previous work has shown that trafficking of this aquaporin is less sensitive to brefeldin A (BFA), an inhibitor of Golgi apparatus function.The soluble zymogen aleurain undergoes complex glycosylation in transit through the Golgi and is cleaved on arrival in the LV. Using a modified aleurain without vacuolar targeting determinants as a reporter protein, possible targeting signals in BP-80 and alpha TIP were investigated1xIntegral membrane protein sorting to vacuoles in plant cells: evidence for two pathways. Jiang, L. and Rogers, J.C. J. Cell Biol. 1998; 143: 1183–1199Crossref | PubMed | Scopus (156)See all References1. The resulting localization was determined biochemically, and constructs with modified C-termini were localized by immunofluoresence.In transgenic tobacco, a reporter protein with the TMD and CTD from BP-80 received complex glycan modification, underwent vacuolar processing and colocalized with tobacco BP-80 homologues at or near the LV. The glycan modification and processing were BFA sensitive, so the reporter proteins travelled from the ER to the LV via the Golgi apparatus.By contrast, swapping the BP-80 TMD or CTD for the respective domain of alpha TIP prevented the normal glycan and proteolytic processing. Furthermore, colocalization of the reporter with BP-80 homologues was abolished, and it now colocalized with alpha TIP in a non-ER compartment, presumably the PSV.The results suggest that the alpha TIP CTD has active targeting information for the PSV, and that the route to the PSV branches from that to the LV in the ER. Since trafficking to the yeast vacuole and mammalian lysosome-like compartments has recently been shown to involve multiple routes, it will be interesting to see whether mammalian and yeast cells have pathways analogous to these emerging pathways in plants.