We have isolated cDNAs encoding human myristoyl-CoA:protein N-myristoyltransferase (NMT, EC 2.3.1.97) by complementing the nmtl-181 mutation of Saccharomyces cerevisiae, which causes temperature-sensitive myristic acid auxotrophy. Human NMT is derived from a single-copy gene, contains 416 amino acids, is 44% identical to S. cerevisiae NMT (yeast NMT), and can complement the lethal phenotype of an nmtl null mutation. Human and yeast NMTs have overlapping yet distinct protein substrate specificities as judged by a coexpression system that reconstitutes protein N-myristoylation in Escherichia coli. Both enzymes contain a glycine five residues from the C terminus. Gly----Asp or Lys mutagenesis in these orthologous NMTs produces marked reductions in their activities in E. coli as well as temperature-sensitive myristic acid auxotrophy in S. cerevisiae. These results indicate highly conserved structure-function relationships in vivo and underscore the usefulness of these functional assays for identifying factors that regulate protein N-myristoylation in mammalian systems.