MEMBRANE TOPOLOGY OF THE 12- AND THE 25-KDA SUBUNITS OF THE MAMMALIANSIGNAL PEPTIDASE COMPLEX

The cleavage of signal sequences of secretory and membrane proteins by the signal peptidase complex occurs in the lumen of the endoplasmic r eticulum. Mammalian signal peptidase consists of five subunits. Four h ave been cloned, SPC18, SPC21, SPC22/23, and SPC25, of which all but S PC25 have been demonstrated to be single-spanning membrane proteins ex posed to the lumen of the endoplasmic reticulum. We have determined th e cDNA sequence of the remaining 12-kDa subunit (SPC12) as well as the membrane topologies of SPC12 and SPC25 in rough microsomes. Both poly peptides span the membrane twice with their N and C termini facing the cytosol and contain only very small, if any, lumenal domains. Therefo re, SPC12 and SPC25 are likely to be involved in processes other than the enzymatic cleavage of the signal sequence.