Protein sorting to the vacuolar membrane.

The vacuolar membrane (tonoplast) of plant cells contains a polytopic integral membrane protein with six membrane-spanning domains and cytoplasmically oriented amino-terminal and carboxy-terminal domains. This protein, tonoplast intrinsic protein (TIP), is a member of the membrane intrinsic protein (MIP) family of proteins, a family of channel proteins found in a variety of organisms. In bean seeds, alpha-TIP is synthesized on the rough endoplasmic reticulum and its transport to the tonoplast is mediated by the secretory system. In this study, we report that a polypeptide segment that includes the sixth membrane domain and the cytoplasmic tail of 18 amino acids of alpha-TIP is sufficient to target the reporter protein phosphinotricine acetyltransferase to the tonoplast of stably transformed tobacco cells. To determine if the carboxy-terminal cytoplasmic tail of alpha-TIP contains important tonoplast targeting information, a deletion construct lacking the 15 carboxy-terminal amino acids was introduced for transient expression in tobacco cells; we found that the slightly truncated protein still accumulated in the tonoplast. From these results, we concluded that a transmembrane domain of a tonoplast protein probably contains sufficient information for transport to the tonoplast. Whether such transport occurs by bulk flow or involves specific cellular machinery remains to be determined.