Electrochemical Potential-driven Transporters - Class 3-- Secondary carrier-type facilitators. Transport systems are included in this category if they utilize a carrier-mediated process to catalyze uniport (a single species is transported by facilitated diffusion in a process not coupled to the utilization of a primary source of energy), antiport (two or more species are transported in opposite directions in a tightly coupled process not directly linked to a form of energy other than chemiosmotic energy) and/or symport (two or more species are transported together in the same direction in a tightly coupled process not directly linked to a form of energy other than chemiosmotic energy). These systems are usually stereospecific. Solute:solute countertransport is a characteristic feature of secondary carriers.

Porters(uniporters,symporters,antiporters)-- Transport systems are included in this subclass if they utilize a carrier-mediated process to catalyze uniport (a single species is transported either by facilitated diffusion or in a membrane potential-dependent process if the solute is char ged), antiport (two or more species are transported in opposite directions in a tightly coupled process, not coupled to a direct form of energy other than chemiosmotic energy) and/or symport (two or more species are transported together in th e same direction in a tightly coupled process, not coupled to a direct form of energy other than chemiosmotic energy).

2.A.37 The Monovalent Cation:Proton Antiporter-2 Family-- The CPA2 family is a moderately large family (over 100 sequenced members) from bacteria, archaea and eukaryotes. Among the functionally well-characterized members of the family are (1) the KefB/KefC K+ efflux proteins of E. coli which may be capable of catalyzing both K+/H+ antiport and K+ uniport, depending on conditions (Bakker et al., 1987; Booth et al., 1996; Munro et al., 1991), (2) the Na+/H+ antiporter of Enterococcus hirae(Waser et al., 1992) and (3) the K+/H+ antiporter of S. cerevisiae. It has been proposed that under normal physiological conditions, these proteins may function by essentially the same mechanism (Reizer et al., 1992). KefC and KefB of E. coli are responsible for glutathione-gated K+ efflux (Ferguson et al., 1993, 1997). Each of these proteins consists of a transmembrane hydrophobic N-terminal domain, and a less well-conserved C-terminal hydrophilic domain. Each protein interacts with a second protein encoded by genes that overlap the gene encoding the primary transporter. The KefC ancillary protein is YabF while the KefB ancillary protein is YheR. These ancillary proteins stimulate transport activity about 10-fold (Miller et al., 2000). These proteins are important for cell survival during exposure to toxic metabolites, possibly because they can release K+, allowing H+ uptake. Activation of the KefB or KefC K+ efflux system only occurs in the presence of glutathione and a reactive electrophile such as methylglyoxal or N-ethylmaleimide. Formation of the methylglyoxal-glutathione conjugate, S-lactoylglutathione, is catalyzed by glyoxalase I, and S-lactoylglutathione activates KefB and KefC (MacLean et al., 1998). H+ uptake (acidification of the cytoplasm) accompanying or following K+ efflux may serve as a further protective mechanism against electrophile toxicity (Booth et al., 1996; Ferguson et al., 1993, 1997; Stumpe et al., 1996).

  1 MNNSVTSNGT HEFVCEAWLG SSSGGLLRGD DPLKYSTPLL LLLISLVSSL SSVFQALLRP
 61 LANVDFVTQI LAGIFLGPSA LGQNIDLVKK LFNTRSYFII ESFEAISFMF ISYISTAQVD
121 MGVIKRGGKL AIINGLSLFL FPYVVGAIAC TVITSNIRGT VAKNNPEQLH NLLTNQSVVY
181 FQVAYSVLSN LKMLNSEPGR LALSSIMVAN CFGWGFFLLL ITFDSFLHQN YSKTTYLPTF
241 TKVLLLVGIV VVCRPIFNWI VKRTPEGKKL KASHLCTICV MLCTATFLSE TVGFPYVVGS
301 VALGLVTPKT PPFGTGLTDK IGSFCYAVLM PCYVIGIGNK VDFFSFNLRD IISLEFLIFT
361 ISAAKFASIV LPSLYFQVPI SHAVIVGFIV CIQGIYDVQI FKQLLNYKNI SHEAFGIMVI
421 SAMVHSTIFT AIVKNLYGWV QRKHITYRRQ TVQHYEPNKP LKILTCFYHR ETVPPILTVL
481 ELSTCPSSAS SHSIVSVNLE ELEQNNVPLL IQHHPGHNDE SSTSSSRRDQ ISKAFEKFRS
541 GHDLQENVSV ECFTAVAPSK TMHEDVCALA FEKETDLIIF GMADGTAAER RLCRNVRNAS
601 PSSVAVLMDQ GRLPDFKNMG TAMKNGSMRI NICSIFLGGA DDRETLAFAV RMTNQPYVNL
661 TVLKLVDGEN VSHLNDVVEK RLDFRTIEKF RQDTMNKHNV ALREEASDLV NLLREEGNNY
721 DLIMVGIRHE KSFEVLQGLS VWSEIEELGE IGDLLVSRDL KLSASVLAVQ QQLSSVVEEV