Spc1 regulates the signal peptidase-mediated processing of membrane proteins

Abstract

Signal peptidase (SPase) cleaves the signal sequences (SSs) of secretory precursors. It contains an evolutionarily conserved membrane protein subunit, Spc1 that is dispensable for the catalytic activity of SPase, and its role remains unknown. In the present study, we investigated the function of yeast Spc1. First, we set up an in vivo SPase cleavage assay using secretory protein CPY variants with SSs modified in the n and h regions. When comparing the SS cleavage efficiencies of these variants in cells with or without Spc1, we found that signal-anchored sequences became more susceptible to cleavage by SPase without Spc1. Further, SPase-mediated processing of model membrane proteins was enhanced in the absence of but reduced upon overexpression of Spc1. Spc1 was co-immunoprecipitated with proteins carrying uncleaved signal-anchored or transmembrane (TM) segments. Taken together, these results suggest that Spc1 protects TM segments from SPase action, thereby sharpening substrate selection for SPase and acting as a negative regulator for the SPase-mediated processing of membrane proteins.