Identification of the NH2-terminal blocking group of NADH-cytochrome b5 reductase as myristic acid and the complete amino acid sequence of the membrane-binding domain.

The NH2-terminal blocking group of the membrane-binding domain of NADH-cytochrome b5 reductase has been deduced as myristic (n-tetradecanoyl) acid. This fatty acid was identified by gas chromatography of the digest of the NH2-terminal tetrapeptide of cytochrome b5 reductase. Fast atom bombardment and direct chemical ionization mass spectroscopy of the underivatized NH2-terminal tetrapeptide confirmed the presence of myristic acid, identified its linkage to the NH2 terminus and established CH3(CH2)12-CO-Gly-Ala-Gln-Leu as the NH2-terminal sequence. In addition, the complete amino acid sequence of the membrane-binding domain of cytochrome b5 reductase is also reported. The finding of a myristic acyl chain on the NH2-terminal segment, comprised of hydrophobic amino acid residues, implies that the function of the myristate group may be other than simply to anchor the reductase to the microsomal membrane. This post-translational modification, presumably in the endoplasmic reticulum, may selectively stabilize a particular membrane structure and orientation that optimally facilitates electron transport on the cytosolic surface of this membrane organelle.