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). Dispensable for SPC enzymatic activity (By similarity). Component of the signal peptidase complex (SPC) composed of a catalytic subunit SEC11 and three accessory subunits SPCS1, 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. Belongs to the SPCS1 family.