ALTERNATIVE SPLICING OF CIC-6 (A MEMBER OF THE CIC CHLORIDE-CHANNEL FAMILY) TRANSCRIPTS GENERATES 3 TRUNCATED ISOFORMS ONE OF WHICH, CIC-6C, IS KIDNEY-SPECIFIC

ClC-6 is a protein that structurally belongs to the family of ClC-type chloride channels. We now report the identification of three addition al ClC-6 isoforms that are truncated because of alternative splicing. We have isolated, from human K562 cells, four types of ClC-6 cDNAs tha t encode four distinct ClC-6 protein isoforms. ClC-6a (869 amino acids ) corresponds to the previously published ClC-6 protein [Brandt and Je ntsch (1995) FEES Lett. 377, 15-20] and it has a canonical ClC structu re. However, ClC-6b (320 amino acids), ClC-6c (353 amino acids) and Cl C-6d (308 amino acids) are truncated at their C-termini. Hydropathy-pl ot analysis indicates that the shortened isoforms contain maximally fo ur (ClC-6b and -6d) or seven (ClC-6c) transmembrane domains. Sequence analysis of a human genomic ClC-6 fragment indicates that the cDNA var iability arises from alternative splicing at two different positions: the first alternative site consists of an intron flanked by two altern ative donor sites and two alternative acceptor sites, the second being due to an exon that is optionally included or excluded. Reverse-trans cription-PCR analysis of ClC-6 expression in human cell lines and tiss ues shows that the majority (83 %) of ClC-6 mRNAs consists of ClC-6a o r ClC-6c messengers. Furthermore, in a mouse tissue panel, the ClC-6a mRNA has a relatively broad tissue expression pattern, since it could be detected in brain, kidney, testis, skeletal muscle, thymus and panc reas. In contrast, expression of ClC-6c is more restricted, since it w as only detected in kidney.