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. Specifically cleaves N-terminal signal peptides that contain a hydrophobic alpha-helix (h-region) shorter than 18-20 amino acids. 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. Within the complex, interacts with SPCS2 and SPCS3. 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. 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.