The gene for hSK4, a novel human small conductance calcium-activated potassium channel, or SK channel, has been identified and expressed in Chinese hamster ovary cells. In physiological saline hSK4 generates a conductance of approximately 12 pS, a value in close agreement with that of other cloned SK channels. Like other members of this family, the polypeptide encoded by hSK4 contains a previously unnoted leucine zipper-like domain in its C terminus of unknown function. hSK4 appears unique, however, in its very high affinity for Ca2+ (EC50 of 95 nM) and its predominant expression in nonexcitable tissues of adult animals. Together with the relatively low homology of hSK4 to other SK channel polypeptides (approximately 40% identical), these data suggest that hSK4 belongs to a novel subfamily of SK channels.

We have isolated a novel gene, hKCa4, encoding an intermediate conductance, calcium-activated potassium channel from a human lymph node library. The translated protein comprises 427 amino acids, has six transmembrane segments, S1-S6, and a pore motif between S5 and S6. hKCa4 shares 41-42% similarity at the amino acid level with three small conductance calcium-activated potassium channels cloned from brain. Northern blot analysis of primary human T lymphocytes reveals a 2.2-kilobase transcript that is highly up-regulated in activated compared with resting cells, concomitant with an increase in KCa current. hKCa4 transcript is also detected by Northern blots or by polymerase chain reaction in placenta, prostate, thymus, spleen, colon, and many cell lines of hematopoietic origin. Patch-clamp recordings of hKCa4-transfected HEK 293 cells reveal a large voltage-independent, inwardly rectifying potassium current that is blocked by externally applied tetraethylammonium (Kd = 30 +/- 7 mM), charybdotoxin (Kd = 10 +/- 1 nM), and clotrimazole (Kd = 387 +/- 34 nM), but is resistant to apamin, iberiotoxin, kaliotoxin, scyllatoxin (Kd > 1 microM), and margatoxin (Kd > 100 nM). Single hKCa4 channels have a conductance of 33 +/- 2 picosiemens in symmetrical potassium solutions. The channel is activated by intracellular calcium (Kd = 270 +/- 8 nM) with a highly cooperative interaction of approximately three calcium ions per channel. These properties of the cloned channel are very similar to those reported for the native KCa channel in activated human T lymphocytes, indicating that hKCa4 encodes this channel type.

>sp|O15554|KCN4_HUMAN Intermediate conductance calcium-activated potassium channel protein 4 (SK4) (KCa4) (IK1) (IKCa1) (Putative Gardos channel) - Homo sapiens (Human).
MGGDLVLGLGALRRRKRLLEQEKSLAGWALVLAGTGIGLMVLHAEMLWFGGCSWALYLFLVKCTISISTFLLLCLIVAFH
AKEVQLFMTDNGLRDWRVALTGRQAAQIVLELVVCGLHPAPVRGPPCVQDLGAPLTSPQPWPGFLGQGEALLSLAMLLRL
YLVPRAVLLRSGVLLNASYRSIGALNQVRFRHWFVAKLYMNTHPGRLLLGLTLGLWLTTAWVLSVAERQAVNATGHLSDT
LWLIPITFLTIGYGDVVPGTMWGKIVCLCTGVMGVCCTALLVAVVARKLEFNKAEKHVHNFMMDIQYTKEMKESAARVLQ
EAWMFYKHTRRKESHAARRHQRKLLAAINAFRQVRLKHRKLREQVNSMVDISKMHMILYDLQQNLSSSHRALEKQIDTLA
GKLDALTELLSTALGPRQLPEPSQQSK