Myristoyl CoA:protein N-myristoyltransferase activities from rat liver and yeast possess overlapping yet distinct peptide substrate specificities

A variety of eukaryotic viral and cellular proteins possesses an NH₂-terminal N-myristoylglycine residue important for their biological functions. Recent studies of the primary structural requirements for peptide substrates of the enzyme responsible for this modification in yeast demonstrated that residues 1, 2, and 5 play a critical role in enzyme:ligand interactions (Towler, D.A., Adams, S.P., Eubanks, S.R., Towery, D.S., Jackson-Machelski, E., Glaser, L., and Gordon, J.I. (1987b) Proc. Natl. Acad. Sci. U.S.A. 84, 2708-2812). This was determined by examining as substrates a series of synthetic peptides whose sequences were systematically altered from a 'parental' peptide derived from the known N-myristoylprotein bovine heart cyclic AMP-dependent protein kinase (A kinase) catalytic subunit. We have now extended these studies in order to examine structure/activity relationships in the COOH-terminal regions of octapeptide substrates of yeast N-myristoyltransferase (NMT). The interaction between yeast NMT and the side chain of residue 5 in peptide ligands is apparently sterically constrained, since Thr⁵ is unable to promote the very high affinity binding observed with a Ser⁵ substitution. A substrate hexapeptide core has been defined which contains much of the information necessary for recognition by this lower eukaryotic NMT. Addition of COOH-terminal basic residues to this hexapeptide enhances peptide binding, while COOH-terminal acidic residues destabilize NMT:ligand interactions. Based on the results obtained from our in vitro studies of over 80 synthetic peptides and yeast NMT, we have identified a number of potential N-myristoylproteins from searches of available protein databases. These include hepatitis B virus pre-S1, human SYN-kinase, rodent G(i)α, and bovine transducin-α. Peptides corresponding to the NH₂-terminal sequences of these proteins and several known N-myristoylproteins were assayed using yeast NMT as well as partially purified rat liver NMT. While a number of the synthetic peptides exhibited similar catalytic properties with the yeast and mammalian enzymes, surprisingly, the SYN-kinase, G(i)α, and transducin-α peptides were N-myristoylated by rat NMT but not by yeast NMT. This suggests that either multiple NMT activities exist in rat liver or the yeast and rodent enzymes have similar but distinct peptide substrate specificities.