Bicarbonate-induced phosphorylation of p270 protein in mouse sperm by cAMP-dependent protein kinase.

Signaling by cAMP-dependent protein kinase (PKA) plays an important role in the regulation of mammalian sperm motility. However, it has not been determined how PKA signaling leads to changes in motility, and specific proteins responsible for these changes have not yet been identified as PKA substrates. Anti-phospho-(Ser/Thr) PKA substrate antibodies detected a sperm protein with a relative molecular weight of 270,000 (p270), which was phosphorylated within 1 min after incubation in a medium supporting capacitation. Phosphorylation of p270 was induced by bicarbonate or a cAMP analog, but was blocked by the PKA inhibitor H-89, indicating that p270 is likely a PKA substrate in sperm. In addition, phosphorylation of p270 was inhibited by stearated peptide st-Ht31, suggesting that p270 is phosphorylated by PKA associated with an A-kinase anchoring protein (AKAP). AKAP4 is the major fibrous sheath protein of mammalian sperm and tethers regulatory subunits of PKA to localize phosphorylation events. Phosphorylation of p270 occurred in sperm lacking AKAP4, suggesting that AKAP4 is not involved directly in the phosphorylation event. Phosphorylated p270 was enriched in fractionated sperm tails and appeared to be present in multiple compartments including a detergent-resistant membrane fraction. PKA phosphorylation of p270 within 1 min of incubation under capacitation conditions suggests that this protein may have an important role in the initial signaling events that lead to the activation and subsequent hyperactivation of sperm motility.