A 2ND MAMMALIAN N-MYRISTOYLTRANSFERASE

N-terminal myristoylation is a cotranslational lipid modification comm on to many signaling proteins that often serves an integral role in th e targeting and/or function of these proteins. Myristoylation is catal yzed by an enzyme activity, N-myristoyltransferase (NMT), which transf ers myristic acid from myristoyl coenzyme A to the amino group of a pr otein's N-terminal glycine residue. While a single human NMT cDNA has been isolated and characterized (hNMT-1), biochemical evidence has ind icated the presence of several distinct NMTs in vivo, often varying in either apparent molecular weight and/or subcellular distribution. We now report the cloning and characterization of a second, genetically d istinct human NMT (hNMT-2), as well as the isolation of the respective mouse NMT homologue for each human enzyme. The mouse and human versio ns of each NMT are highly homologous, displaying greater than 95% amin o acid sequence identity. Comparisons between the NMT-1 and NMT-2 prot eins revealed reduced levels of sequence identity (76-77%), indicating that NMT-1 and NMT-2 comprise two distinct families of N-myristoyltra nsferases. Transient transfection of either the hNMT-1 or hNMT-2 cDNA into COS-7 cells resulted in the expression of high levels of NMT enzy me activity. Both hNMT-1 and hNMT-2 were found to myristoylate several commonly studied peptide substrates with similar, hut distinguishable , relative selectivities. Western analysis revealed that while hNMT-2 appeared as a single 65-kDa protein in transfected COS-7 cells, hNMT-1 was processed to provide four distinct protein isoforms ranging from 49 to 68 kDa in size. Collectively, these studies demonstrate a hereto fore unappreciated level of genetic complexity underlying the enzymolo gy of N-terminal myristoylation and suggest that the specific inhibiti on or regulation of either NMT in vivo may in turn allow for the selec tive control of particular myristoylation-dependent cellular functions .