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.

Sis1 (1-337), which lacks the dimerization motif, exhibited severe defects in chaperone activity, but could regulate Hsp70 ATPase activity. Thus, dimer formation is critical for Sis1 chaperone function. We propose that the Sis1 cleft functions as a docking site for the Hsp70 peptide-binding domain and that Sis1-Hsp70 interaction serves to facilitate the efficient transfer of peptides from Sis1 to Hsp70.

The mechanisms by which Hsp40 functions as a molecular chaperone to recognize and bind non-native polypeptides is not understood. We have identified a peptide substrate for Ydj1, a member of the type I Hsp40 from yeast. The structure of the Ydj1 peptide binding fragment and its peptide substrate complex was determined to 2.7 A resolution. The complex structure reveals that Ydj1 peptide binding fragment forms an L-shaped molecule constituted by three domains. The domain I exhibits a similar protein folds as domain III while the domain II contains two Zinc finger motifs. The peptide substrate binds Ydj1 by forming an extra beta strand with domain I of Ydj1. The Leucine residue in the middle of the peptide substrate GWLYEIS inserts its side chain into a hydrophobic pocket formed on the molecular surface of Ydj1 domain I. The Zinc finger motifs located in the Ydj1 domain II are not in the vicinity of peptide substrate binding site.

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Matrix protein (M1) of influenza virus is a bifunctional protein that mediates the encapsidation of RNA-nucleoprotein cores into the membrane envelope. It is therefore required that M1 binds both membrane and RNA simultaneously. The X-ray crystal structure of the N-terminal portion of type A influenza virus M1-amino acid residues 2-158-has been determined at 2.08 A resolution at pH 4.0. The protein forms a dimer. A highly positively charged region on the dimer surface is suitably positioned to bind RNA while the hydrophobic surface opposite the RNA binding region may be involved in interactions with the membrane. The membrane-binding hydrophobic surface could be buried or exposed after a conformational change.