Interactions of human sperm acrosomal protein SP-10 with the acrosomal membranes

The interaction of the human acrosomal protein SP-10 with the acrosomal membranes was analyzed by the ability of Triton X-114 (TX-114) and other agents to release SP-10 from the acrosome. Treatment of human sperm with TX-114 revealed a pool of SP-10 that was released by TX-114 and a pool of SP-10 that was TX-114-resistant. TX-114-resistant SP-10 was associated with the equatorial segment and with TX-114-resistant portions of the acrosomal matrix and the inner acrosomal membrane. Phase partitioning of TX-114-released and TX-114-resistant SP-10 pools showed that both were hydrophilic, indicating that these pools consist of proteins that are peripherally associated with, rather than integral to, the acrosomal membranes. Sequential treatments of human sperm with various agents showed that repeated washes with TX-114 or 1.5 M NaCl had little or no effect on TX-114-resistant SP-10, whereas treatment with a chaotropic salt (150 mM sodium thiocyanate) and buffers at pH extremes (pH 2.0 and 10.0) completely released this pool of SP-10 from the acrosome. Together the results suggest that SP-10 is a hydrophilic peripheral acrosomal membrane protein that may be associated with a TX-114-resistant "anchor."