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 and sperm. Isoform 2, but not isoform 1, is present at the erythrocyte membrane (at protein level).