SEC61P IS ADJACENT TO NASCENT TYPE-I AND TYPE-II SIGNAL-ANCHOR PROTEINS DURING THEIR MEMBRANE INSERTION

We have identified membrane components which are adjacent to type I an d type II signal-anchor proteins during their insertion into the membr ane of the ER. Using two different cross-linking approaches a 37-38-kD nonglycosylated protein, previously identified as P37 (High, S., D. G orlich, M. Wiedmann, T. A. Rapoport, and B. Dobberstein. 1991. J. Cell Biol. 113:35-44), was found adjacent to all the membrane inserted nas cent chains used in this study. On the basis of immunoprecipitation, t his ER protein was shown to be identical to the recently identified ma mmalian Sec61 protein. Thus, Sec61p is the principal cross-linking par tner of both type I and type II signal-anchor proteins during their me mbrane insertion (this work), and of secretory proteins during their t ranslocation (Gorlich, a, S. Prehn, E. Hartmann, K.-U. Kalies, and T. A. Rapoport. 1992. Cell. 71:489-503). We propose that membrane protein s of both orientations, and secretory proteins employ the same ER tran slocation sites, and that Sec61p is a core component of these sites.