CYCLIC NUCLEOTIDE-GATED CHANNELS ON THE FLAGELLUM CONTROL CA2+ ENTRY INTO SPERM

Cyclic nucleotide-gated (CNG) channels are key elements of cGMP- and c AMP-signaling pathways in vertebrate photoreceptor cells and in olfact ory sensory neurons, respectively. These channels form heterooligomeri c complexes composed of at least two distinct subunits (alpha and beta ). The alpha subunit of cone photoreceptors is also present in mammali an sperm. Here we identify one short and several long less abundant tr anscripts of beta subunits in testis, The alpha and beta subunits are expressed in a characteristic temporal and spatial pattern in sperm an d precursor cells. In mature sperm. the ct subunit is observed along t he entire flagellum, whereas the short beta subunit is restricted to t he principal piece of the flagellum. These findings suggest that diffe rent forms of CNG channels coexist in the flagellum. Confocal microsco py in conjunction with the Ca2+ indicator Fluo-3 shows that the CNG ch annels serve as a Ca2+ entry pathway that responds more sensitively to cGMP than to cAMP. Assuming that CNG channel subtypes differ in their Ca2+ permeability, dissimilar localization of alpha and beta subunits may give rise to a pattern of Ca2+ microdomains along the flagellum, thereby providing the structural basis for control of flagellar bendin g waves.