Modifications of cysteine residues in the solution and membrane-associated conformations of phosphatidylinositol transfer protein have differential effects on lipid transfer activity.
The alpha isoforms of mammalian phosphatidylinositol transfer protein (PITP) contain four conserved Cys residues. In this investigation, a series of thiol-modifying reagents, both alkylating and mixed disulfide-forming, was employed to define the accessibility of these residues and to evaluate their role in protein-mediated intermembrane phospholipid transport. Isolation and analysis of chemically modified peptides and site-directed mutagenesis of each Cys residue to Ala were also performed. Soluble, membrane-associated, and denatured preparations of wild-type and mutant rat PITPs were studied. Under denaturing conditions, all four Cys residues could be detected spectrophotometrically by chemical reaction with 4,4'-dipyridyl disulfide or 5,5'-dithiobis(2-nitrobenzoate). In the native protein, two of the four Cys residues were sensitive to some but not all thiol-modifying reagents, with discrimination based on the charge and hydrophobicity of the reagent and the conformation of the protein. With the soluble conformation of PITP, achieved in the absence of phospholipid vesicles, the surface-exposed Cys(188) was chemically modified without consequence to lipid transfer activity. Cys(188) exhibited an apparent pK(a) of 7.6. The buried Cys(95), which constitutes part of the phospholipid substrate binding site, was covalently modified upon transient association of PITP with a membrane surface. The Cys-to-Ala mutations showed that neither Cys(95) nor Cys(188) was essential for lipid transfer activity. However, chemical modification of Cys(95) resulted in the loss of lipid transfer activity. These results demonstrate that the Cys residues of PITP can be assigned to several different classes of chemical reactivity. Of particular interest is Cys(95), whose sulfhydryl group becomes exposed to modification in the membrane-associated conformation of PITP. Furthermore, the inhibition of PITP activity by thiol-modifying reagents is a result of steric hindrance of phospholipid substrate binding.