Mechanism of interaction of PITPα with membranes: Conformational changes in the C-terminus associated with membrane binding

Eukaryotic phosphatidylinositol transfer proteins (PITPs) are composed predominantly of small (∼32 kDa) soluble proteins that bind and transfer a single phospholipid, normally phosphatidylinositol or phosphatidycholine. Two forms, PITPα and PITPβ, which share approximately 80% amino acid sequence similarity, are known. Rat PITPα 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.