Human sperm CRISP2 is released from the acrosome during the acrosome reaction and re-associates at the equatorial segment.

Sperm CRISP2 has been proposed to be involved in sperm-egg fusion. After the acrosome reaction, it appears at the equatorial segment (EqS) of human sperm; the mechanism underlying the appearance of CRISP2 at the EqS remains unknown, though. Here, we provide evidence showing the re-association of sperm acrosomal CRISP2 at the EqS during the acrosome reaction. Results showed that F-actin is not involved in the relocalization of CRISP2. We found that basic, but not acidic, conditions can solubilize CRISP2 from sperm cells, suggesting that CRISP2 is a component of the acrosome and that it is released from the acrosome during the acrosome reaction. Purified, biotinylated human sperm acrosomal CRISP2 binds to the EqS of acrosome-reacted sperm in a dose-dependent manner, revealing that CRISP2 detected at the EqS of acrosome-reacted sperm comes from the population stored in the acrosome. The association of CRISP2 at the EqS is very strong, and does not depend on ionic interactions or intermolecular disulfide bonds. Interestingly, the restriction of CRISP2 at the EqS was diminished when EGTA was present in the media, indicating that Ca(2+) is required for maintaining CRISP2 at the EqS. This study supports the possibility that CRISP2 may help modify the EqS membrane to make this domain fusion-competent.

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