Enzymatic activity of sperm proprotein convertase is important for mammalian fertilization

Abstract

Proprotein convertase subtilisin/kexin 4 (PCSK4) is implicated for sperm fertilizing ability, based on studies using Pcsk4-null mice. Herein we demonstrated proprotein convertase (PC) activity in intact sperm and acrosomal vesicles. To determine whether this activity was important for sperm fertilizing ability, a peptide inhibitor was designed based on PCSK4 prodomain sequence (proPC4 75-90 ), which contains its primary autocatalytic cleavage site. ProPC4 75-90 inhibited recombinant PCSK4's activity with a K i value of 5.4μM, and at 500μM, it inhibited sperm PC activity almost completely. Treatment of sperm with proPC4 75-90 inhibited their egg fertilizing ability in a dose dependent manner. Correlation between sperm PC activity and fertilizing ability showed a high co-efficient value ( > 0.9), indicating the importance of sperm PC activity in fertilization. In particular, sperm PC activity was important for capacitation and zona pellucida (ZP)-induced acrosome reaction, since proPC4 75-90 -treated sperm showed markedly decreased rates in these two events. These results were opposite to those observed in Pcsk4-null sperm, which contained higher PC activity than wild type sperm, possibly due to overcompensation by PCSK7, the other PCSK enzyme found in sperm. ADAM2 (45kDa), a sperm plasma membrane protein, involved in sperm-egg plasma membrane interaction, was also processed into a smaller form (27kDa) during capacitation at a much reduced level in proPC4 75-90 -treated sperm. This result suggested that ADAM2 may be a natural substrate of sperm PCSK4 and its cleavage by the enzyme during acrosome reaction may be relevant to the fertilization process. © 2011 Wiley-Liss, Inc.