Essential 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:9148931, PubMed:9148930). Essential for the SPC catalytic activity, possibly by stabilizing and positioning the active center of the complex close to the lumenal surface (PubMed:9148931, PubMed:9148930). Essential for viability (PubMed:9148931, PubMed:9148930). Component of the signal peptidase complex (SPC) composed of a catalytic subunit SEC11 and three accessory subunits SPC1, SPC2 and SPC3 (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 SEC11 (PubMed:10206957). SPC associates with the translocon complex (PubMed:9148931). Present with 6840 molecules/cell in log phase SD medium. Belongs to the SPCS3 family.