2.A.51 The Chromate Ion Transporter (CHR) Family
Homologues of the CHR family have been identified in bacteria, eukaryotes and archaea. Two Bacillus homologues are half length with six putative TMSs each (Nies et al., 1998). These two proteins together (2.A.51.1.5), but not singly, cause chromate resistance due to formation of a heterodimer and consequent chromate efflux (Díaz-Magaña et al., 2009). Half-sized homologues are found in several bacteria. The functionally characterized chromate efflux pumps of P. aeruginosa and A. eutrophus are plasmid-encoded. They are about 400 amino acyl residues long with 10 putative transmembrane α-helical spanners (TMSs). They arose by a tandem internal gene duplication event from a putative 6 TMS primordial precursor, but the first two TMSs in the A. entrophus ChrA protein have lost their hydrophobic character (Nies et al., 1998).
In a more recent study, (Diaz-Perez et al., 2007), 77 duplicated 'bidomains' (BDs) and 58 unduplicated 'monodomains' (MDs) were identified and analyzed. The MDs clustered separately from the N-terminal BDs, and both clustered separately from the C-terminal BDs. This suggests that the MDs, possibly present in inverted orientation in the membrane, may have a unique structure and mode of action (Diaz-Perez et al., 2007).
The membrane topology of the ChrA protein of P. aeruginosa was conducted using lacZ and phoA translational fusions (Jiminez-Mejia et al., 2006). A 13 TMS topology was predicted with the N-terminus in the cytoplasm and the C-terminus in the periplasm. Predicted TMSs 1-6 proved to be homologous to predicted TMSs 8-13, but with opposite orientation in the membrane.
Synechococcus sp. PCC7942 bears an endogenous 50 Kb plasmid-encoded, sulfur-regulated CHR homologue that apparently confers chromate sensitivity (rather than chromate resistance) when grown in media containing a low sulfate concentration. This protein, designated SrpC, may be a sulfate uptake permease that can also transport chromate, but this possibility has not been established. ChrA of P. aeruginosa is a secondary carrier which might function by chromate uniport, chromate:H+ antiport, or chromate:anion antiport. It catalyzes CrO4 efflux with a Km of 80 μM. SO4= and MoO4= inhibit efflux but arsenate and vanadate do not inhibit (Pimentel et al., 2002). A pmf dependency is likely since valinomycin, nigericin and protonophores such as CCCP inhibit. Ramírez-Díaz et al. (2008) have published a review concerning the mechanisms of bacterial resistance to chromium compounds. In Neurosporal crassa, a chromate uptake transporter, Chr-1, a member of the Chr family, has been described (Flores-Alvarez et al. 2012).
The generalized transport reaction catalyzed by prokaryotic CHRs may be:
CrO42- (in) [+ nH+ (out)] → CrO42- (out) [+ nH+ (in)]
The generalized transport reaction catalyzed by SrpC or Chr-1 may be:
SO42- or CrO42- (out) + nH+ (out) → SO42- or CrO42- (in) + nH+ (in)
References:
Uncharacterized protein of 984 aas and 13 TMSs, 1 N-terminal and 12 C-terminal.
UP of Phytophthora fragariae
Uncharacterized protein of 544 aas and 11 TMSs in a 4 + 7 TMS arrangement.
UP of Cavenderia fasciculata
Chromate transporter of 513 aas and 10 - 12 TMSs (Hori et al. 2014).
Chromate transporter of Klebsormidium nitens
Chromate-resistance efflux pump, ChrA of 416 aas and 11 or 12 TMSs. The chrA gene of Pseudomonas aeruginosa plasmid pUM505 encodes ChrA, which confers resistance to chromate by the energy-dependent efflux of chromate ions (Aguilera et al. 2004). Chromate-sensitive mutants were mostly point mutations affected amino acids clustered in the N-terminal half of ChrA, altering either cytoplasmic regions or transmembrane segments, and replaced residues moderately to highly conserved in ChrA homologs. PhoA and LacZ translational fusions were used to confirm the membrane topology at the N-terminal half of the ChrA protein (Aguilera et al. 2004).
Bacteria
ChrA of Pseudomonas aeruginosa plasmid pUM505
Chromate efflux protein Chr3N/Chr3C, or YwrB/YwrA (Díaz-Magaña et al., 2009). Each protein has 5 TMSs, but they have antiparallel orientation in the membrane: Chr3N has the C-terminus in the cytoplasm while Chr3C has the C-terminus facing the extracellular medium (Martínez-Valencia et al. 2012).
Bacteria
YwrB/YwrA or Chr3N/Chr3C of Bacillus subtilis
Chr3N (O05216)
Chr3C (O05215)
Putative two component chromate porter. The two proteins each have 5 TMSs and are of 195 and 187 aas in length.
Spirochaetes
Two component transporter of Treponema denticola
Chromate resistance protein, ChrA, of 389 aas and 11 TMSs.
ChrA of E. coli
Chromate efflux transporter, ChrA, of 420 aas and 11 TMSs.
ChrA of Euryarchaeota archaeon TMED141 (marine metagenome)
Chromate sensitivity protein, a chromate uptake protein, Chr-1, of 560 aas and 9 - 11 TMSs. N. crassa CHR-1 takes up chromate in a sulfate-independent fashon (Flores-Alvarez et al. 2012).
Chr-1 of Neurospora crassa