STRUCTURE OF THE MEMBRANE-BINDING DOMAIN OF CTP-PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE

It has been proposed that the domain of the regulatory enzyme, CTP:pho sphocholine cytidylyltransferase, which mediates reversible binding of the enzyme to membranes, is an amphipathic alpha-helix of approximate ly 60 amino acid residues and that this domain is adjacent to the puta tive active site domain of this enzyme. Circular dichroism indicated t hat the secondary structures of two overlapping peptides spanning this region were predominantly alpha-helical in the presence of PG vesicle s or sodium dodecyl sulfate micelles. Interproton distances were obtai ned from two-dimensional NMR spectroscopic measurements to solve the s tructures of these two peptides. The C-terminal 22 amino acid peptide segment (corresponding to Val267-Ser288) was a well-defined alpha-heli x over its length. The N-terminal 33-mer (corresponding to Asn236-Glu2 68) was composed of an alpha-helix from Glu243 to Lys266, a well-struc tured bend of about 50 degrees at Tyr240-His241-Leu242, and an N-termi nal four-residue helix. It is proposed that the three residues involve d in generating the bend act as the hinge between the catalytic and re gulatory domains. The nonpolar faces of the 33-mer and 22-mer were int errupted by Ser260, Ser271, and Ser282. These residues may serve to li mit the hydrophobicity and facilitate reversible and lipid-selective m embrane binding. The hydrophobic faces of the helices were flanked by a set of basic amino acid residues on one side and basic amino acid re sidues interspersed with glutamates on the other. The disposition of t hese side chains gives clues to the basis for the specificities of the se peptides for anionic surfaces.