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:27499293, PubMed:34388369). Essential for the SPC catalytic activity, possibly by stabilizing and positioning the active center of the complex close to the lumenal surface (By similarity). {ECO:0000250|UniProtKB:Q12133, ECO:0000269|PubMed:27499293, ECO:0000269|PubMed:34388369}. (Microbial infection) Plays an important role in virion production of flaviviruses such as West Nile virus, Japanese enchephalitis virus, Dengue virus type 2 and Yellow Fever virus. . This is the function of ENSG00000129128 (SPCS3, signal peptidase complex subunit 3).