PROTEIN-PROTEIN INTERACTIONS INVOLVING T4 PHAGE-CODED DEOXYCYTIDYLATEDEAMINASE AND THYMIDYLATE SYNTHASE

The enzymes deoxycytidylate deaminase (EC 3.5.4.12) and thymidylate sy nthase (EC 2.1.1.45) are functionally associated with one another, sin ce they catalyze sequential reactions, In T4 coliphage infection the t wo enzymes are found in dNTP synthetase, a multienzyme complex for deo xyribonucleotide biosynthesis, Protein-protein interactions involving the phage-coded forms of these two enzymes have been explored in three experiments that use the respective purified protein as an affinity l igand. First, an extract of radiolabeled T4 proteins was passed throug h a column of immobilized enzyme (either dTMP synthase or dCMP deamina se), and the specifically bound proteins were identified, Second, two mutant form of dCMP deaminase (H90N and H94N), altered in presumed zin c-binding sites, were analyzed similarly, with the results suggesting that some, but not all, interactions require normal structure near the catalytic site, Third, affinity chromatography using either enzyme as the immobilized ligand, revealed interactions between the two purifie d enzymes in the absence of other proteins, In these experiments we no ted a significant effect of dCTP, an allosteric modifier of dCMP deami nase, upon the interactions.