Misplacement of the amino-terminal positive charge in the prepro-alpha-factor signal peptide disrupts membrane translocation in vivo.

We have identified a series of mutations in the signal peptide of yeast prepro-alpha-factor which specifically attenuate translocation across the endoplasmic reticulum membrane in vivo. In prepro-alpha-factor-somatostatin hybrids, transposition of the amino-terminal tripeptide from wild-type NH2-Met-Arg-Phe to NH2-Met-Phe-Lys or NH2-Met-Phe-Arg causes a 45-75% reduction in the efficiency of membrane translocation. This is evidenced by the intracellular accumulation of unglycosylated, signal-containing precursors which are membrane-associated and are exposed to the cytosol. Surprisingly, abolition of the single positive charge by replacing arginine with phenylalanine has little effect on translocation into the endoplasmic reticulum. We conclude that the presence of an amino-terminal positive charge is not necessary for efficient targeting or translocation; however, misplacement by one position markedly disrupts translocation without affecting targeting. These mutations thus define an early stage of membrane interaction that is sensitive to local charge effects. Furthermore, our data suggest that post-translational translocation, signal cleavage, and core glycosylation of these polypeptides may occur to a significant extent in vivo.