Proprotein convertase involved in the processing of hormone and other protein precursors at sites comprised of pairs of basic amino acid residues. 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 (PubMed:9192653, PubMed:16371590, PubMed:19342015, PubMed:21302280). 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 (By similarity). May also participate in folliculogenesis in the ovaries (PubMed:11164898). The proPCSK4 form interacts with HSPA5; the interaction takes place at the endoplasmic reticulum. Expressed abundantly in the testis since postnatal Day 16 (PubMed:1372895, PubMed:16371590, PubMed:21302280). In testis, strongly detected in round and elongated spermatids as well as spermatocytes. Also observed in residual bodies engulfed by Sertoli cells at spermatogenic stages VIII and IX (PubMed:16371590). In ovaries, expressed in macrophage-like cells of the ovarian theca, interstitium and corpora lutea (PubMed:11164898). Detected only after the 20th day of postnatal development. Mainly expressed in the round spermatids. Expressed mainly in the early stages of spermiogenesis. N-glycosylated. Synthesized in the endoplasmic reticulum as a zymogen, is matured by autocatalytic cleavage between the prodomain and the catalytic domain. Knockout males show impaired fertility (PubMed:9192653). Sperm from mutants exhibit accelerated capacitation, precocious acrosome reaction, reduced binding to egg zona pellucida, and impaired fertilizing ability (PubMed:16371590). They have abnormal acrosome formation during spermatogenesis (PubMed:22357636). Sperm proteins are hyper-tyrosine phosphorylated during capacitation (PubMed:19342015). In females, the percent of successful mating is comparable to that of their PCSK4 +/- female littermates, however the average litter size of the former is half that of the latter. Knockout ovaries treated with gonatropin are generally smaller, less hyperemic and with fewer corpora lutea than wild type. This difference is associated with a 20-fold lower level of circulating progesterone (PubMed:9192653). Belongs to the peptidase S8 family. Furin subfamily.