NOVEL DEVELOPMENTALLY-REGULATED PHOSPHOINOSITIDE BINDING-PROTEINS FROM SOYBEAN WHOSE EXPRESSION BYPASSES THE REQUIREMENT FOR AN ESSENTIAL PHOSPHATIDYLINOSITOL TRANSFER PROTEIN IN YEAST

Phosphatidylinositol transfer proteins (PITPs) have been shown to play important roles in regulating a number of signal transduction pathway s that couple to vesicle trafficking reactions, phosphoinositide-drive n receptor-mediated signaling cascades, and development, While yeast a nd metazoan PITPs have been analyzed in some detail, plant PITPs remai n entirely uncharacterized. We report the identification and character ization of two soybean proteins, Ssh1p and Ssh2p, whose structural gen es were recovered on the basis of their abilities to rescue the viabil ity of PITP-deficient Saccharomyces cerevisiae strains. We demonstrate that, while both Ssh1p and Ssh2p share similar to 25% primary sequenc e identity with yeast PITP, these proteins exhibit biochemical propert ies that diverge from those of the known PITPs, Ssh1p and Ssh2p repres ent high-affinity phosphoinositide binding proteins that are distingui shed from each other both on the basis of their phospholipid binding s pecificities and by their substantially non-overlapping patterns of ex pression in the soybean plant. Finally, we show that Ssh1p is phosphor ylated in response to various environmental stress conditions, includi ng hyperosmotic stress. We suggest that Ssh1p may function as one comp onent of a stress response pathway that serves to protect the adult pl ant from osmotic insult.