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'''PHOSPHATIDYLINOSITOL TRANSFER PROTEIN SEC14P FROM SACCHAROMYCES CEREVISIAE'''

The yeast phosphatidylinositol-transfer protein (Sec14) catalyses exchange of phosphatidylinositol and phosphatidylcholine between membrane bilayers in vitro. In vivo, Sec14 activity is essential for vesicle budding from the Golgi complex. Here we report a three-dimensional structure for Sec14 at 2.5 A resolution. Sec14 consists of twelve alpha-helices, six beta-strands, eight 3(10)-helices and has two distinct domains. The carboxy-terminal domain forms a hydrophobic pocket which, in the crystal structure, is occupied by two molecules of n-octyl-beta-D-glucopyranoside and represents the phospholipid-binding domain. This pocket is reinforced by a string motif whose disruption in a sec14 temperature-sensitive mutant results in destabilization of the phospholipid-binding domain. Finally, we have identified an unusual surface helix that may play a critical role in driving Sec14-mediated phospholipid exchange. From this structure, we derive the first molecular clues into how a phosphatidylinositol-transfer protein functions.

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