Proprotein convertase involved in the processing of hormone and other protein precursors at sites comprised of pairs of basic amino acid residues (By similarity). In males, important for ADAM2 processing as well as other acrosomal proteins with roles in fertilization and critical for normal fertilization events such as sperm capacitation, acrosome reaction and binding of sperm to zona pellucida (By similarity). Also plays a role in female fertility, involved in the regulation of trophoblast migration and placental development, may be through the proteolytical processing and activation of proteins such as IGF2 (PubMed:16040806). May also participate in folliculogenesis in the ovaries (By similarity). The proPCSK4 form interacts with HSPA5; the interaction takes place at the endoplasmic reticulum. Placenta. Localized predominantly in the cytotrophoblast layer of trophoblast cells during the first trimester of pregnancy, and to the syncytiotrophoblast and stroma cells during the third trimester. N-glycosylated. Synthesized in the endoplasmic reticulum as a zymogen, is matured by autocatalytic cleavage between the prodomain and the catalytic domain. Belongs to the peptidase S8 family. Furin subfamily.