Mechanism of interaction of PITPalpha with membranes: conformational changes in the C-terminus associated with...

Eukaryotic phosphatidylinositol transfer proteins (PITPs) are composed predominantly of small ( approximately 32 kDa) soluble proteins that bind and transfer a single phospholipid, normally phosphatidylinositol or phosphatidycholine. Two forms, PITPalpha and PITPbeta, which share approximately 80% amino acid sequence similarity, are known. Rat PITPalpha was labeled at specific single reactive Cys residues with I-AEDANS and used to examine PITP-membrane interactions. Upon binding to phospholipid vesicles, PITP labeled with AEDANS at the C-terminus, a region postulated to be involved in membrane binding, shows significant decreases in both steady-state and dynamic fluorescence anisotropy. In contrast, PITPs labeled with AEDANS at sites located distal to the C-terminus show increases in both steady-state and dynamic anisotropy. These results suggest that interaction of PITP with membrane surfaces leads to significant alterations in conformation and perhaps melting of the C-terminal helix.