Cloning and characterization of KCC3 and KCC4, new members of the cation-chloride cotransporter gene family

The K+-Cl- cotransporters (KCCs) belong to the gene family of electroneutra l cation-chloride cotransporters, which also includes two bumetanide-sensit ive Na+-K+-2Cl(-) cotransporters and a thiazide sensitive Na+-Cl- cotranspo rter. We have cloned cDNAs encoding mouse KCC3, human KCC3, and human KCC4, three new members of this gene family. The KCC3 and KCC4 cDNAs predict pro teins of 1083 and 1150 amino acids, respectively. The KCC3 and KCC4 protein s are 65-71% identical to the previously characterized transporters KCC1 an d KCC2, with which they share a predicted membrane topology, The four KCC p roteins differ at amino acid residues within key transmembrane domains and in the distribution of putative phosphorylation sites within the amino- and carboxyl-terminal cytoplasmic domains. The expression of mouse KCC3 in Xen opus laevis oocytes reveals the expected functional characteristics of a KCl- cotransporter: Cl--dependent uptake of Rb-86(+) which is strongly activ ated by cell swelling and weakly sensitive to furosemide, A direct function al comparison of mouse KCC3 to rabbit KCC1 indicates that KCC3 has a much g reater volume sensitivity. The human KCC3 and KCC4 genes are located on chr omosomes 5p15 and 15q14, respectively. Although widely expressed, KCC3 tran scripts are the most abundant in heart and kidney, and KCC4 is expressed in muscle, brain, lung, heart, and kidney, The unexpected molecular heterogen eity of K+-Cl- cotransport has implications for the physiology and pathophy siology of a number of tissues.