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 (By similarity). 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 (By similarity). 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. This ensures the selectivity of the complex towards h-regions shorter than 18-20 amino acids (By similarity). SPC associates with the translocon complex (By similarity). The C-terminal short (CTS) helix is essential for catalytic activity. 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. Belongs to the peptidase S26B family. Extended N-terminus.