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). (Microbial infection) Plays an important role in infection by flaviviruses such as West Nile virus and Dengue virus type 2. Component of the signal peptidase complex (SPC) composed of a catalytic subunit twr/SEC11 and three accessory subunits Spase12/SPCS1, Spase25/SPCS2 and Spase22-23/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. (Microbial infection) RNAi-mediated knockdown reduces infection by West Nile virus (WNV) and Dengue virus type 2 (DENV-2). Belongs to the SPCS1 family.