Proprotein convertase 4 (PCSK4) is a member of a family of proprotein convertases that convert inactive precursor proteins into their mature and active forms. PCSK4 is expressed by testicular germ cells and localizes to the sperm acrosome, suggesting roles in fertilization. Mice lacking PCSK4 exhibit a profound fertility defect; yet, to date, few substrates for PCSK4 are known. In this study, two-dimensional differential in-gel electrophoresis analysis was carried out in order to identify proteins that are altered in spermatozoa from PCSK4 null mice. Herein, we report that the sperm fertilization molecule acrosin-binding protein (ACRBP)/sp32, which normally undergoes processing from a 58.5 kDa precursor to a 27.5 kDa mature form, is not proteolytically processed in PCSK4 null mice and thus may be a substrate for PCSK4. However, analysis of the ACRBP sequence did not show a strong consensus site for convertase cleavage, suggesting that ACRBP processing may require the activity of a yet unknown enzyme that itself may be a PCSK4 substrate. Further analysis of spermatozoa from the PCSK4 null mice showed that proacrosin did not undergo autoactivation, supporting a role for the mature form of ACRBP in the regulation of proacrosin conversion into different acrosin isoforms. Finally, examination of ACRBP localization revealed a previously undetected morphological defect in the head/acrosomes of spermatozoa from PCSK4 null mice. Taken together, these results demonstrate that the fertility defect in the PCSK4 null mice may in part be due to altered ACRBP protein processing as well as abnormalities in the sperm head/acrosome.