Acts as a cofactor for complement factor I, a serine protease which protects autologous cells against complement-mediated injury by cleaving C3b and C4b deposited on host tissue. May be involved in the fusion of the spermatozoa with the oocyte during fertilization. Also acts as a costimulatory factor for T-cells which induces the differentiation of CD4+ into T-regulatory 1 cells. T-regulatory 1 cells suppress immune responses by secreting interleukin-10, and therefore are thought to prevent autoimmunity (By similarity).
Subcellular locations: Cytoplasmic vesicle, Secretory vesicle, Acrosome inner membrane
Inner acrosomal membrane of spermatozoa.
Present in blood, liver, lung and testes. Isoform 2, but not isoforms 1 or 3, is present at the erythrocyte membrane (at protein level). Whereas isoforms 2/3 are ubiquitous, isoform 1 is expressed only in testes, brain and heart.