Structure of the Membrane Binding Domain of CTP:Phosphocholine Cytidylyltransferase

It has been proposed that the domain of the regulatory enzyme, CTP:phosphocholine cytidylyltransferase, which mediates reversible binding of the enzyme to membranes, is an amphipathic α-helix of approximately 60 amino acid residues and that this domain is adjacent to the putative active site domain of this enzyme. Circular dichroism indicated that the secondary structures of two overlapping peptides spanning this region were predominantly α-helical in the presence of PG vesicles or sodium dodecyl sulfate micelles. Interproton distances were obtained from two-dimensional NMR spectroscopic measurements to solve the structures of these two peptides. The C-terminal 22 amino acid peptide segment (corresponding to Val267−Ser288) was a well-defined α-helix over its length. The N-terminal 33-mer (corresponding to Asn236−Glu268) was composed of an α-helix from Glu243 to Lys266, a well-structured bend of about 50° at Tyr240−His241−Leu242, and an N-terminal four-residue helix. It is proposed that the three residues involved in generating the bend act as the hinge between the catalytic and regulatory domains. The nonpolar faces of the 33-mer and 22-mer were interrupted by Ser260, Ser271, and Ser282. These residues may serve to limit the hydrophobicity and facilitate reversible and lipid-selective membrane binding. The hydrophobic faces of the helices were flanked by a set of basic amino acid residues on one side and basic amino acid residues interspersed with glutamates on the other. The disposition of these side chains gives clues to the basis for the specificities of these peptides for anionic surfaces.