Species-specific differences among KCNMB3 BK β3 auxiliary subunits:: Some β3 N-terminal variants may be primate-specific subunits

The KCNMB3 gene encodes one of a family of four auxiliary beta subunits found in the mammalian genome that associate with Slo1 alpha subunits and regulate BK channel function. In humans, the KCNMB3 gene contains four N-terminal alternative exons that produce four functionally distinct beta 3 subunits, beta 3a-d. Three variants, beta 3a-c, exhibit kinetically distinct inactivation behaviors. Since investigation of the physiological roles of BK auxiliary subunits will depend on studies in rodents, here we have determined the identity and functional properties of mouse beta 3 variants. Whereas beta 1, beta 2, and beta 4 subunits exhibit 83.2%, 95.3%, and 93.8% identity between mouse and human, the mouse beta 3 subunit, excluding N-terminal splice variants, shares only 62.8% amino acid identity with its human counterpart. Based on an examination of the mouse genome and screening of mouse cDNA libraries, here we have identified only two N-terminal candidates, beta 3a and beta 3b, of the four found in humans. Both human and mouse beta 3a subunits produce a characteristic use-dependent inactivation. Surprisingly, whereas the h beta 3b exhibits rapid inactivation, the putative m beta 3b does not inactivate. Furthermore, unlike h beta 3, the m beta 3 subunit, irrespective of the N terminus, mediates a shift in gating to more negative potentials at a given Ca(2+) concentration. The shift in gating gradually is lost following patch excision, suggesting that the gating shift involves some regulatory process dependent on the cytosolic milieu. Examination of additional genomes to assess conservation among splice variants suggests that the putative m beta 3b N terminus may not be a true orthologue of the h beta 3b N terminus and that both beta 3c and beta 3d appear likely to be primate-specific N-terminal variants. These results have three key implications: first, functional properties of homologous beta 3 subunits may differ among mammalian species; second, the specific physiological roles of homologous beta 3 subunits may differ among mammalian species; and, third, some beta 3 variants may be primate-specificion channel subunits.