Phylogenetic analysis of the chromate ion transporter (CHR) superfamily

ChrA is a membrane protein that confers resistance to the toxic ion chromate through the energy-dependent chromate efflux from the cytoplasm. In the protein databases, ChrA is a member of the chromate ion transporter (CHR) superfamily, composed of at least several dozens of members, distributed in the three domains of life. The aim of this work was to perform a phylogenetic analysis of the CHR superfamily. An exhaustive search for ChrA homologous proteins was carried out at the National Center for Biotechnology Information database. One hundred and thirty-five sequences were identified as members of the CHR superfamily [77 long-chain sequences, or bidomains (LCHR), and 58 short-chain sequences, or monodomains (SCHR)], organized mainly as tandem pairs of genes whose resultant proteins probably possess oppositely oriented membrane topology. LCHR sequences were split into amino and carboxyl domains, and the resultant domains were aligned with the SCHR proteins. A phylogenetic tree was reconstructed using four different methods, obtaining similar results. The domains were grouped into three clusters: the SCHR proteins cluster, the amino domain cluster of LCHR proteins and the carboxyl domain cluster of LCHR proteins. These results, as well as differences in the genomic context of CHR proteins, enabled the proteins to be sorted into two families (SCHR and LCHR), and 10 subfamilies. Evidence was found suggesting an ancient origin of LCHR proteins from the fusion of two SCHR protein-encoding genes; however, some secondary events of fusion and fission may have occurred later. The separate distribution of the LCHR and SCHR proteins, differences in the genomic context in both groups and the fact that chromate transport has been demonstrated only in LCHR proteins suggest that the CHR proteins comprise two or more paralogous groups in the CHR superfamily.