Family 1.A.35 - The CorA Metal Ion Transporter Family
Family ID: 53372
The MIT family, also called the CorA family, is a large and diverse
family with sequenced members in Gram-positive and Gram-negative
bacteria, blue-green bacteria, archaea, and yeast. Functionally
characterized proteins include the Mg2+-Co2+-Ni2+ CorA permeases
of Salmonella typhimuriumand E. coli, a CorA homologue in Methanococcus
janaschii, aluminum-resistance Mg2+ transport permeases (AlR1p
and AlR2p) of Saccharomyces cerevisiaeand a manganese-resistance
permease (MnR2p) of S. cerevisiae. Most members of the MIT family
are between 300 and 400 amino acyl residues in length and possess
two or three putative transmembrane a-helical spanners (TMSs).
It seems clear that some homologues have two and others have three
TMSs (M. Maguire, personal communication). The greatest degree
of conservation between homologues is found in TMSs 2 and 3 of
the enteric bacterial proteins. The yeast metal resistance proteins,
which are 850-900 amino acyl residues in length, also exhibit
two or three putative TMSs. Overexpression of the yeast proteins,
AlR1p and MnR2p, overcomes toxicity to aluminum and manganese,
respectively.
The CorA permeases
of S. typhimuriumand E. colimediate both influx and efflux of
Mg2+. They transport Mg2+, Co2+ and Ni2+. Mg2+ is transported
with an apparent KM of 20-30 mM. The archaeal CorA protein is
functionally similar to the CorA homologues of enteric bacteria.
The yeast proteins appear to exhibit broad specificity transporting
a wide range of di- and trivalent metal cations. In this respect,
and also with respect to topology, MIT family members resemble
channel proteins.
The transport
mode and energy coupling mechanism(s) are poorly understood. An
oligomeric, possibly pentameric (M. Maguire, personal communication)
structure for the transporter complex is inferred, and a channel-type
mechanism rather than a carrier-type mechanism is favored due
to its high capacity and its "gating" properties. CorA
of S. typhimuriumis a high capacity transporter that is constitutively
synthesized. MIT family members may serve as the major Mg2+ influx
systems for many prokaryotes.
