Are isoforms of capacitating Na K -ATPase localized to sperm head rafts?

Capacitation begins in the sperm head plasma membrane (HPM). Membrane rafts could house signaling molecules, but although these specialized microdomains have been microscopically visualized in sperm heads, rafts have been isolated for study only from homogenized whole sperm or tails, never purified HPM. Sodium/potassium ATPase (Na K -ATPase) is a membrane-bound signaling protein that induces capacitation in bull sperm in response to the steroid hormone ouabain, and its subunit isoforms α1, α3, β1, β2, and β3 are known in HPM.

Interaction of ouabain and progesterone on induction of bull sperm capacitation.

The endogenous steroid hormone ouabain induces capacitation of bull sperm acting through its receptor Na/K-ATPase on the sperm plasma membrane. Progesterone (P4) is believed to act through the sperm membrane P4 receptor (mPR) to induce non-genomic signalling leading to capacitation and/or acrosome reaction (AR) in the sperm of some species, but the exact nature of this receptor molecule on bull sperm is not known. In amphibian oocytes, P4 acts through the low-affinity ouabain binding site on Na/K-ATPase to induce signalling highly reminiscent of ouabain's signalling that initiates capacitation.