We report the identification and characterisation of a novel splice-variant of the human SK3 small-conductance Ca²⁺-activated K⁺ channel (SK_Ca).

Using the GenBank Expressed Sequence Tag (EST) database to identify SK_Ca-related sequences, a full-length cDNA was obtained from Raji cell RNA by a combination of 5â and 3â rapid amplification of cDNA ends (5â and 3â RACE). The predicted translation product differed from hSK3 in the N-terminus, with a novel sequence of five amino acids replacing the conventional N-terminal and first transmembrane segment. RT-PCR indicated that the novel sequence was expressed in kidney, lung and trachea among a panel of normal human tissues (obtained from Clontech), a distribution distinct from that of conventional hSK3. Subcloning the novel coding sequence into pcDNA3.1 mammalian expression vector and transfection of HEK cells resulted in expression of a V5 epitope-tagged protein. Immunofluorescence studies indicated that anti-V5 labelling was confined to the intracellular compartment, presumably the endoplasmic reticulum, and labelling was not evident at the plasmalemma. Co-expression with conventional hSK3 neither rescued the novel variant nor allowed its plasmalemmal expression, nor did the variant inhibit the robust plasmalemmal expression of hSK3. Current-voltage relationships, derived from whole-cell patch-clamp recordings (1 µM free calcium pipette solution) of cells transfected with the novel variant or co-transfected with hSK3, were indistinguishable from those recorded in the absence of the novel variant.

These data suggest that the novel hSK3 splice variant transcript identified in normal human tissues does express at the protein level when transfected into HEK cells. However, under these expression conditions, the protein does not form a plasmalemmal K⁺ channel or associate with hSK3 subunits.