Differential display polymerase chain reaction has been used to isolate genes regulated in vascular endothelial cells by the angiogenic factor vascular endothelial cell growth factor (VEGF). Analysis of one of the bands consistently up-regulated by VEGF led us to the identification of a cDNA from a human umbilical vein endothelial cell library that is 77% identical to the human K+-Cl- cotransporter1 (KCC1). We have referred to the predicted protein as K+-Cl- cotransporter 3 (KCC3). Hydrophobicity analysis of the KCC3 amino acid sequence showed an almost identical pattern to KCC1, suggesting 12 membrane-spanning segments, a large extracellular loop with potential N-glycosylation sites, and cytoplasmic N- and C-terminal regions. The KCC3 mRNA was highly expressed in brain, heart, skeletal muscle, and kidney, showing a distinct pattern and size from KCC1 and KCC2. The KCC3 mRNA level in endothelial cells increased on treatment with VEGF and decreased with the proinflammatory cytokine tumor necrosis factor alpha, whereas KCC1 mRNA levels remained unchanged. Stable overexpression of KCC3 cDNA in HEK293 cells produced a glycoprotein of approximately 150 kDa, which was reduced to 120 kDa by glycosidase digestion. An increased initial uptake rate of 86Rb was seen in clones with high KCC3 expression, which was dependent on extracellular Cl- but not Na+ and was inhibitable by the loop diuretic agent furosemide. The KCC3 genomic localization was shown to be 15q13 by fluorescence in situ hybridization. Radiation hybrid analysis placed KCC3 within an area associated with juvenile myoclonic epilepsy. These results suggest KCC3 is a new member of the KCC family that is under distinct regulation from KCC1.

The K+-Cl- cotransporters (KCCs) belong to the gene family of electroneutral cation-chloride cotransporters, which also includes two bumetanide-sensitive Na+-K+-2Cl- cotransporters and a thiazide-sensitive Na+-Cl- cotransporter. 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 proteins of 1083 and 1150 amino acids, respectively. The KCC3 and KCC4 proteins are 65-71% identical to the previously characterized transporters KCC1 and KCC2, with which they share a predicted membrane topology. The four KCC proteins 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 Xenopus laevis oocytes reveals the expected functional characteristics of a K+Cl- cotransporter: Cl--dependent uptake of 86Rb+ which is strongly activated by cell swelling and weakly sensitive to furosemide. A direct functional comparison of mouse KCC3 to rabbit KCC1 indicates that KCC3 has a much greater volume sensitivity. The human KCC3 and KCC4 genes are located on chromosomes 5p15 and 15q14, respectively. Although widely expressed, KCC3 transcripts are the most abundant in heart and kidney, and KCC4 is expressed in muscle, brain, lung, heart, and kidney. The unexpected molecular heterogeneity of K+-Cl- cotransport has implications for the physiology and pathophysiology of a number of tissues.

>sp|Q9UHW9|S126_HUMAN Solute carrier family 12 member 6 (Electroneutral potassium-chloride cotransporter 3) (K-Cl cotransporter 3) - Homo sapiens (Human).
MHPPETTTKMASVRFMVTPTKIDDIPGLSDTSPDLSSRSSSRVRFSSRESVPETSRSEPMSEMSGATTSLATVALDPPSD
RTSHPQDVIEDLSQNSITGEHSQLLDDGHKKARNAYLNNSNYEEGDEYFDKNLALFEEEMDTRPKVSSLLNRMANYTNLT
QGAKEHEEAENITEGKKKPTKTPQMGTFMGVYLPCLQNIFGVILFLRLTWVVGTAGVLQAFAIVLICCCCTMLTAISMSA
IATNGVVPAGGSYFMISRALGPEFGGAVGLCFYLGTTFAAAMYILGAIEIFLVYIVPRAAIFHSDDALKESAAMLNNMRV
YGTAFLVLMVLVVFIGVRYVNKFASLFLACVIVSILAIYAGAIKSSFAPPHFPVCMLGNRTLSSRHIDVCSKTKEINNMT
VPSKLWGFFCNSSQFFNATCDEYFVHNNVTSIQGIPGLASGIITENLWSNYLPKGEIIEKPSAKSSDVLGSLNHEYVLVD
ITTSFTLLVGIFFPSVTGIMAGSNRSGDLKDAQKSIPIGTILAILTTSFVYLSNVVLFGACIEGVVLRDKFGDAVKGNLV
VGTLSWPSPWVIVIGSFFSTCGAGLQSLTGAPRLLQAIAKDNIIPFLRVFGHSKANGEPTWALLLTAAIAELGILIASLD
LVAPILSMFFLMCYLFVNLACALQTLLRTPNWRPRFRYYHWALSFMGMSICLALMFISSWYYAIVAMVIAGMIYKYIEYQ
GAEKEWGDGIRGLSLSAARFALLRLEEGPPHTKNWRPQLLVLLKLDEDLHVKHPRLLTFASQLKAGKGLTIVGSVIVGNF
LENYGEALAAEQTIKHLMEAEKVKGFCQLVVAAKLREGISHLIQSCGLGGMKHNTVVMGWPNGWRQSEDARAWKTFIGTV
RVTTAAHLALLVAKNISFFPSNVEQFSEGNIDVWWIVHDGGMLMLLPFLLKQHKVWRKCSIRIFTVAQLEDNSIQMKKDL
ATFLYHLRIEAEVEVVEMHDSDISAYTYERTLMMEQRSQMLRHMRLSKTERDREAQLVKDRNSMLRLTSIGSDEDEETET
YQEKVHMTWTKDKYMASRGQKAKSMEGFQDLLNMRPDQSNVRRMHTAVKLNEVIVNKSHEAKLVLLNMPGPPRNPEGDEN
YMEFLEVLTEGLERVLLVRGGGSEVITIYS