Differential activation of bovine brain N-myristoyltransferase(s) by a cytosolic activator.

N-Myristoyltransferase (NMT) is an essential eukaryotic enzyme that catalyses the transfer of myristate from myristoyl-CoA to the NH2-terminal glycine residue of a number of important proteins of diverse function. Little is known about the control and regulation of NMT in higher eukaryotes. We have identified a N-myristoyltransferase activator (NAF45) which copurified with one form of bovine brain NMT activity which failed to bind to phosphocellulose column chromatography (NMT.PU) and represented 32 +/- 4% of the total NMT activity applied to this column. The NAF45 and NMT activities were resolved by mono Q column chromatography from the NMT.PU fraction. Resolution resulted in a loss of NMT.PU activity. NAF45 was a nondialysable molecule with an apparent molecular mass of 45 kDa. Reconstitution of the NAF45 and NMT resulted in the recovery of NMT activity, indicating that this form of NMT is dependent on the presence of NAF45. NAF45 was able to stimulate other forms of NMT activity (maximum of 3-4 fold) in a highly cooperative fashion (Hills coefficient of approximately 5.5). Stimulation was in terms of both the rate of myristoylation and the overall production of myristoylpeptide, indicating that the end point of the reaction was shifted in favour of myristoylpeptide production. NAF45 was able to compete with NIP71 for NMT [King, M.J. and Sharma, R.K. (1993) Biochem, J. 291, 635-639], although its affinity appeared lower than the NIP71.