An Arabidopsis VPS45p Homolog Implicated in Protein Transport to the Vacuole1

Abstract

The Sec1p family of proteins is required for vesicle-mediated protein trafficking between various organelles of the endomembrane system. This family includes Vps45p, which is required for transport to the vacuole in yeast (Saccharomyces cerevisiae). We have isolated a cDNA encoding a VPS45 homolog from Arabidopsis thaliana (AtVPS45). The cDNA is able to complement both the temperature-sensitive growth defect and the vacuolar-targeting defect of a yeast vps45 mutant, indicating that the two proteins are functionally related. AtVPS45p is a peripheral membrane protein that associates with microsomal membranes. Sucrose-density gradient fractionation demonstrated that AtVPS45p co-fractionates with AtELP, a potential vacuolar protein sorting receptor, implying that they may reside on the same membrane populations. These results indicate that AtVPS45p is likely to function in the transport of proteins to the vacuole in plants.Soluble proteins that reside in the plant vacuole are initially translocated into the ER lumen and are then transported through the secretory pathway to the TGN, where vacuolar proteins are sorted from those that are to be secreted by virtue of a vacuolar-targeting signal. These signals are presumably recognized by receptor proteins in the TGN that allow transport to the vacuole. Two types of cleavable targeting signals have been identified: C-terminal propeptides and N-terminal propeptides, both of which are removed during deposition in the vacuole. Different mechanisms may be responsible for the transport of proteins containing different classes of signals Robinson and Hinz, 1997).Proteins are transported between organelles of the secretory pathway in vesicles that bud from one compartment and fuse with the next. The mechanism by which transport vesicles containing cargo proteins fuse with a target membrane has begun to be elucidated through a combination of biochemical studies of synaptic vesicle fusion with the presynaptic plasma membrane in mammalian neurons, and in genetic studies of secretion in yeast (Rothman, 1996). Similar sets of proteins were shown to be involved in the highly regulated fusion events in neurotransmission and the constitutive secretory system of yeast.These studies led to the proposal of the SNARE hypothesis to explain how a transport vesicle could be targeted to and fused with the correct organelle out of the many membrane-bound compartments of the cell. It was proposed that certain membrane proteins on the transport vesicle (v-SNAREs) and the target organelle (t-SNAREs) interact with each other and with several soluble proteins to allow docking of the vesicle with the target membrane and to promote vesicle fusion. For example, at the presynaptic membrane, a complex is formed between the v-SNARE synaptobrevin/vesicle-associated membrane protein and the t-SNAREs syntaxin and SNAP-25 (synaptosomeassociated protein of 25 kD), allowing the soluble factors N-ethylmaleimide-sensitive factor and ␣-SNAP (soluble N-ethylmaleimide-sensitive factor attachment protein) to bind. Hy...

The Putative Arabidopsis Homolog of Yeast Vps52p Is Required for Pollen Tube Elongation, Localizes to Golgi, and Might Be Involved in Vesicle Trafficking

“…As observed for POK, most of these genes have been shown to be ubiquitously expressed, but to higher levels in roots (Ahmed et al, 1997;Conceic xão et al, 1997;Bassham and Raikhel, 1998;Rojo et al, 2003). Although many genes homologous to those involved in the yeast Golgi vesicle trafficking have already been identified in Arabidopsis (e.g.…”

“…Although many homologs of these proteins have been identified in plants (e.g. Ahmed et al, 1997; Conceic xão et al, 1997;Bassham and Raikhel, 1998;Sanderfoot et al, 1998;Zheng et al, 1999) Article, publication date, and citation information can be found at www.plantphysiol.org/cgi …”

Plant Cells Are Not Just Green Yeast

“…In yeast, this protein functions in two transport pathways to the vacuole: (a) the carboxypeptidase Y pathway, where it is involved in fusion of TGN-derived vesicles at the PVC and interacts with the prevacuolar t-SNARE Pep12p; and (b) the cytoplasm-to-vacuole pathway, where it is required for the formation of transport intermediates at the TGN and interacts with the TGN t-SNARE Tlg2p. An Arabidopsis cDNA encoding a protein (AtVPS45) with extensive sequence similarity to yeast Vps45p is able to complement the carboxypeptidase Y sorting defects and the temperature-sensitive growth defect of a yeast vps45 mutant (Bassham and Raikhel, 1998). However, AtVPS45 in Arabidopsis resides exclusively on the TGN, not on the PVC, and interacts only with the TGN t-SNAREs and not with t-SNAREs found on the PVC (D.C. Bassham, A.A. Sanderfoot, V. Kovaleva, H. Zheng, and N.V. Raikhel, unpublished data).…”

TNO1 Is Involved in Salt Tolerance and Vacuolar Trafficking in Arabidopsis

“…However, the mechanism of recycling of VSR1 from the PVC to the TGN is not yet fully understood. A complex containing the SNAREs SYP41, SYP61, and VTI12 and the Sec1/Munc18-like protein AtVPS45, a potential regulator of vesicle fusion (Dulubova et al, 2002;Bryant and James, 2003), is found at the TGN in Arabidopsis (Arabidopsis thaliana; Bassham and Raikhel, 1998;. SNAREs in the SYP41 complex, along with another SNARE, YKT61/ 62, were found to be sufficient for vesicle fusion in vitro (Chen et al, 2005).…”