tus, plasma membrane, and vacuole. Soluble proteins containing an N-terminal signal sequence are The syntaxin family of proteins are involved in vesicle co-translationally inserted into the ER lumen and from sorting, docking and fusion in the secretory pathway. here are transported through the secretory system in The discovery of several syntaxin homologues in plants transport vesicles which bud from one compartment and (AtPEP12p, KNOLLE, AtVAM3p) suggested that the fuse with the next. From the ER, proteins are transported general mechanisms of protein trafficking through the through the Golgi cisternae to the trans-Golgi network, secretory pathway also apply to plant cells. The identi- where a major sorting event occurs. Proteins are either fication of another syntaxin homologue, the AtPLP secreted to the outside of the cell (the default pathway) (AtPEP12p-Like Protein) gene, in Arabidopsis thaliana, or they are targeted to the vacuole via a prevacuolar which is not present in yeast, is reported here. The compartment (PVC ), a route that is believed to be putative amino acid sequence of AtPLP shows high receptor-mediated. similarity to AtPEP12p and AtVAM3p, and AtPEP12p antibodies cross-react with in vitro translated AtPLP. Protein sorting to the vacuole The AtPLP gene shows polymorphism between two For a soluble protein to reach the vacuole, it must contain Arabidopsis ecotypes: In ecotype Columbia, it encodes a specific vacuolar sorting signal. Three types of vacuolar a syntaxin homologue lacking the C-terminal trans- sorting signal have been identified. Some proteins contain membrane domain (TMD), whereas in ecotype RLD, a propeptide which comprises the sorting signal and due to a frame shift in the genomic sequence, it results which is removed from the protein upon deposition in in a typical syntaxin-like protein with a C-terminal the vacuole. Several proteins are known to contain a TMD. The distribution pattern of AtPLP mRNA among propeptide at their N-terminus, including sweet potato various tissues is different from that of AtPEP12, sporamin (Matsuoka and Nakamura, 1991) and barley AtVAM3 and KNOLLE mRNA, indicating that the protein aleurain (Holwerda et al., 1992). Deletion and mutagenmay play a novel role in vesicle trafficking in plant esis studies have shown that the N-terminal propeptide is cells.