Export of major cell surface proteins is blocked in yeast secretory mutants

Abstract

The transport of newly synthesized proteins to the yeast cell surface was analyzed by a modification of the technique developed by Kaplan et al. Cells metabolically labeled with 35SO42- are treated with trinitrobenzenesulfonic acid (TNBS) at 0.degree. C under conditions where cell-surface proteins are tagged with trinitrophenol (TNP) but cytoplamsic proteins are not. After fractionation of cells into cell wall, membrane and cytoplasmic samples, and solubilization with SDS [sodium dodecyl sulfate] the tagged proteins are immunoprecipitated with anti-TNP antibody and fixed Staphyloccous aureus cells. Analysis of the precipitates by SDS gel electrophoresis and fluorography reveals 4 major protein species in the cell wall (S1-S4), 7 species in the membrane fraction (M1-M7), and no tagged proteins in the cytoplasmic fraction. Temperature-sensitive mutants defective in secretion of invertase and acid phosphatase (sec mutants) are also defective in transport of the 11 major cell surface proteins at the nonpermissive temperature (37.degree. C). Export of accumulated proteins is restored in an energy-dependent fashion when seCI cells are returned to a permissive temperature (24.degree. C). In wild-type cells the transit time for different surface proteins varies from < 8 min to .apprx. 30 min. The asynchrony is developed at an early stage in the secretory pathway. All of the major cell wall proteins and many of the externally exposed plasma membrane proteins bind to concanavalin A. Inhibition of asparagine-linked glycosylation with tunicamycin does not prevent transport of several surface proteins.