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:3511473, PubMed:8444896, PubMed:14559916). 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 paralog A (SPC-A) composed of a catalytic subunit SEC11A and three accessory subunits SPCS1, SPCS2 and SPCS3 (PubMed:3511473). Within the complex, interacts with SPCS2 and SPCS3 (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 (By similarity). This ensures the selectivity of the complex towards h-regions shorter than 18-20 amino acids (By similarity). 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). Belongs to the peptidase S26B family.