Analysis of the compartmentalization of myristoyl-CoA:protein N-myristoyltransferase in Saccharomyces cerevisiae

Myristoyl-CoA:protein N-myristoyltransferase (NMT) catalyzes the cotranslational, covalent attachment of a rare fatty acid, myristic acid (C14:0), to the amino-terminal glycine residue of a number of eukaryotic proteins involved in cellular growth and signal transduction as well as several viral proteins necessary for assembly-replication. NMT has become a target for both anti-viral and anti-fungal therapy. Analysis of purified Saccharomyces cerevisiae NMT plus yeast strains with conditional lethal nmt1 mutations have provided insights about how this process is regulated in vivo. We have now defined the location of NMT in two strains of S. cerevisiae to better understand the functional and spatial relationships between this enzyme and cellular systems that generate its acyl-CoA and peptide ligands. Western blot studies using an affinity purified antibody raised in rabbits against purified S. cerevisiae NMT indicate that the acyltransferase represents 0.06% of total cellular proteins in an exponentially growing haploid strain with a wild type NMT1 allele. Another strain containing a single, integrated copy of a GAL1/NMT1 fusion gene and a nmt1 null allele had 12-fold higher levels of NMT when grown on galactose-containing media. This increase in NMT production had no detectable effects on growth or cellular morphology. Cell fractionation studies, confocal fluorescence immunocytochemical analysis, and immunogold electron microscopic surveys of fixed, gelatin-embedded cryosections of both strains revealed that NMT is a cytosolic protein that is not associated with cellular membranes (including the endoplasmic reticulum and plasma membrane), the nucleus, mitochondria, Golgi apparatus, or vacuoles. This finding is discussed in light of what is known about the location and activities of enzymes involved in de novo fatty acid biosynthesis and in amino-terminal processing of nascent proteins.