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.19 The Ca2+:Cation Antiporter Family-- Proteins of the CaCA family are found ubiquitously, having been identified in animals, plants, yeast, archaea and divergent bacteria. They exhibit widely divergent sequences, and several have been shown to have arisen by a tandem intragenic duplication event (Saier et al., 1999). The most conserved portions of this repeat element, a1 and a2, are found in TMSs 2-3 and TMSs 7-8 in the model of Iwamoto et al. (1999). These sequences are important for transport function and may form an intramembranous pore/loop-like structure. Members of the CaCA family vary in size from 302 amino acyl residues (Methanococcus jannaschii) to 1199 residues (Bos taurus). Even within the animal kingdom, they vary in size from 461 to 1199 residues. The bacterial and archaeal proteins are in general smaller than the eukaryotic proteins (Chung et al., 2001). They have been suggested to traverse the membrane 9 (mammals) or 10 (bacteria) times as a-helical spanners. The E. coliChaB(YrbG) homologue has been found to have 10 TMSs with both the N- and C-termini localized to the periplasm. Each homologous half of the internally duplicated protein has 5 TMS with opposite orientation in the membrane (Saaf et al., 2001). The mammalian cardiac muscle homologue probably has 9 TMSs. The N-terminus of this protein is believed to be extracellular, while the C-terminus is intracellular (Iwamoto et al., 1999). A large central loop is not required for transport function and plays a role in regulation. In the preferred 9 TMS model for this mammalian protein, the polypeptide chain loops into the membrane after TMS 2 and after TMS 7. The large central loop separates TMS 5 from TMS 6. TMS 2 plus the following loop show sequence similarity to TMS 7 and its loop. TMS 7 may be close to TMSs 2 and 3 in the 3-D structure of the protein (Qui et al., 2001).

  1 MSSISTESSS NLSLLENGGG GSDKPTAETS RRVRRTVSAS SLIRKRSDLK LISRVRWEFM
 61 RRILTNLQEV LLGTKLFILF PAVPLAVVAH RYDCPRAWVF ALSLLGLTPL AERISFLTEQ
121 IAFHTGPTVG GLMNATCGNA TEMIIAILAV GQRKMRIVKL SLLGSILSNL LFVLGTSLFL
181 GGISNLRKHQ SFDPRQGDMN SMLLYLALLC QTLPMIMRFT MEAEEYDGSD VVVLSRASSF
241 VMLIAYLAFL IFHLFSSHLS PPPPPLPQRE DVHDDDVSDK EEEGAVIGMW SAIFWLIIMT
301 LLVALLSDYL VSTIQDAADS WGLSVGFIGI ILLPIVGNAA EHAGAVIFAF RNKLDITLGI
361 ALGSATQIAL FVVPVTVLVA WTMGIEMDLN FNLLETACFA LSILVTSLVL QDGTSNYMKG
421 LVLLLCYVVI AACFFVSNSP STETNTTNHT ITKR