Ca2+-activated K+ channels in human leukemic Jurkat T cells - Molecular cloning, biochemical, and functional characterization

Previous studies have demonstrated the presence of apamin-sensitive, small- conductance Ca2+-activated K+ currents in human leukemic Jurkat T cells. Us ing a combined cDNA and reverse transcriptase-polymerase chain reaction clo ning strategy, we have isolated from Jurkat T cells a 2.5-kilobase cDNA, hS K2, encoding the human isoform of SK2 channels. Northern blot analysis reve als the presence of a 2.5-kilobase hSK2 transcript in Jurkat T cells. While present in various human tissues, including brain, heart, skeletal muscle, kidney, and liver, no hSK2 mRNA could be detected in resting and activated normal human T cells. The hSK2 gene is encoded by 8 exons and could be ass igned to chromosome 5 (q21.2-q22.1). The protein encoded by hSK2 is 579 ami no acids long and exhibits 97% identity with its rat counterpart rSK2. When expressed in Chinese hamster ovary cells, hSK2 produces Ca2+-activated Kcurrents with a unitary conductance of 9.5 pS and a K-0.5 for calcium of 0. 7 muM; hSK2 currents are inhibited by apamin, scyllatoxin, and d-tubocurari ne. Overexpression of the Src family tyrosine kinase p56(lck) in Jurkat cel ls, upregulates SK2 currents by 3-fold. While IKCa channels are transcripti onally induced upon activation of normal human T cells, our results show th at in Jurkat cells SK2 channels are constitutively expressed and down-regul ated following mitogenic stimulation.