Catalytic component of the signal peptidase complex (SPC) which catalyzes the cleavage of N-terminal signal sequences from nascent proteins as they are translocated into the lumen of the endoplasmic reticulum (PubMed:2644273, PubMed:7615509, PubMed:10206957, PubMed:11058593). Specifically cleaves N-terminal signal peptides that contain a hydrophobic alpha-helix (h-region) shorter than 18-20 amino acids (By similarity). Cleavage of hydrophobic, N-terminal signal or leader sequences from secreted and periplasmic proteins. Component of the signal peptidase complex (SPC) composed of a catalytic subunit SEC11 and three accessory subunits SPC1, SPC2 and SPC3 (PubMed:1846444, PubMed:9148931). The complex induces a local thinning of the ER membrane which is used to measure the length of the signal peptide (SP) h-region of protein substrates (By similarity). This ensures the selectivity of the complex towards h-regions shorter than 18-20 amino acids (By similarity). Interacts with SPC3 (PubMed:10206957). SPC associates with the translocon complex (PubMed:1846444, PubMed:9148931). The C-terminal short (CTS) helix is essential for catalytic activity (By similarity). It may be accommodated as a transmembrane helix in the thinned membrane environment of the complex, similarly to the signal peptide in the complex substrates (By similarity). N-glycosylated. Leads to accumulation of core-glycosylated glycoprotein precursors. Present with 3150 molecules/cell in log phase SD medium. Belongs to the peptidase S26B family.