Efficient ER exit and vacuole targeting of yeast Sna2p require two tyrosine-based sorting motifs.

SNA proteins (Sensitive to Na+) form a membrane protein family, which, in the yeast Saccharomyces cerevisiae, is composed of four members, Sna1p/Pmp3p, Sna2p, Sna3p and Sna4p. In this study, we focused on the 79 residue Sna2p protein. We found that Sna2p is localized in the vacuolar membrane. Directed mutagenesis showed that two functional tyrosine motifs YXXO are present in the C-terminal region. Each of these is involved in a different Golgi-to-vacuole targeting pathway : the tyrosine 65 motif is involved in AP-1-dependent targeting, while the tyrosine 75 motif is involved in AP-3-dependent targeting. Moreover, our data suggest that these motifs also play a crucial role in the exit of Sna2p from the ER. Directed mutagenesis of these tyrosines led to a partial redirection of Sna2p to lipid bodies, probably due to a decrease in ER exit efficiency. Sna2p is the first yeast protein in which two YXXO motifs have been identified and both were demonstrated to be functional at two different steps of the secretory pathway, ER exit and Golgi-to-vacuole transport.