Translocation of polypeptide chains across the endoplasmic reticulum membrane is triggered by signal sequences. During translocation of the nascent chain through the membrane, the signal sequence of most secretory and membrane proteins is cleaved off. Cleavage occurs by the signal peptidase complex (SPC) which consists of four subunits in yeast and five in mammals. This family is common to yeast and mammals [1,2].

1: Membrane topology of the 12- and the 25-kDa subunits of the mammalian signal peptidase complex. Kalies KU, Hartmann E; J Biol Chem 1996;271:3925-3929. PMID:8632014

2: The yeast SPC22/23 homolog Spc3p is essential for signal peptidase activity. Meyer HA, Hartmann E; J Biol Chem 1997;272:13159-13164. PMID:9148931