Ca²⁺-activated K⁺ channels in human leukemic Jurkat T cells

Previous studies have demonstrated the presence of apamin-sensitive, small-conductance Ca²⁺-activated K⁺ currents in human leukemic Jurkat T cells. Using a combined cDNA and reverse transcriptase-polymerase chain reaction cloning strategy, we have isolated from Jurkat T cells a 2.5-kilobase cDNA, hSK2, encoding the human isoform of SK2 channels. Northern blot analysis reveals 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 assigned to chromosome 5 (q21.2-q22.1). The protein encoded by hSK2 is 579 amino acids long and exhibits 97% identity with its rat counterpart rSK2. When expressed in Chinese hamster ovary cells, hSK2 produces Ca²⁺-activated K⁺ currents with a unitary conductance of 9.5 pS and a K_0.5 for calcium of 0.7 μM; hSK2 currents are inhibited by apamin, scyllatoxin, and d-tubocurarine. Overexpression of the Src family tyrosine kinase p56^lck in Jurkat cells, upregulates SK2 currents by 3-fold. While IKCa channels are transcriptionally induced upon activation of normal human T cells, our results show that in Jurkat cells SK2 channels are constitutively expressed and down-regulated following mitogenic stimulation.