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conferenceseries
.com
Volume 10, Issue 8 (Suppl)
J Proteomics Bioinform, an open access journal
ISSN: 0974-276X
Structural Biology 2017
September 18-20, 2017
9
th
International Conference on
Structural Biology
September 18-20, 2017 Zurich, Switzerland
Subin Kim et al., J Proteomics Bioinform 2017, 10:8(Suppl)
DOI: 10.4172/0974-276X-C1-0101
Structural insights into the elevator-like mechanism of the sodium/citrate symporter CitS
Subin Kim
2
, Ji Won Kim
1
, Haerim Lee
1
, Songwon Kim
2
, Jie-Oh Lee
1
and
Mi Sun Jin
2
1
KAIST, Korea
2
SGIST, Korea
T
he 2-HCT family of transporters generally translocate molecules with a 2-hydroxycarboxylate motif (HO-CR1R2-COO-),
such as citrate, malate and lactate across the plasma membrane, and activity is tightly coupled to energy from a sodium
or proton gradient.
Klebsiella pneumoniae
CitS (
Kp
CitS) is the best-characterized model system, which has been purified in
detergent and characterized in a reconstituted state. It plays a key role for citrate uptake to ultimately produce ATP in anaerobic
fermentative process. Single-molecule fluorescence spectroscopy study provided an evidence for formation of homodimeric
Kp
CitS. Analysis of hydropathy profiles and rich biochemical data suggested that it consists of 11 transmembrane helixes with
two putative reentrant loops. Mutational studies showed that R428, which is strictly conserved in transporters of the 2-HCT
family, is critical for interaction with one of the carboxylate groups of citrate. Analysis of data from kinetics experiments
demonstrated that
Kp
CitS carries citrate followed by binding of sodium ion. However, there are conflicting data regarding
exact stoichiometry. The structure of
Kp
CitS was studied extensively by electron crystallography, providing a glimpse of its
global structure. The crystal structure of a homologous symporter from
Salmonella enterica
(SeCitS) recently revealed that it
forms an asymmetric dimer, and that each protomer embeds a substrate translocation pathway at the interface between the
transport and the dimerization domains. That structure provided the first high resolution view of a member of the 2-HCT
family; however, many details in the transport cycle remained unanswered.
Biography
Subin Kim graduated from Chonnam National University in 2014 and completed her MS from Gwangju Institute of Science and Technology (GIST) in 2016 and she
joined as PhD candidate in School of Life Sciences at Gwangju Institute of Science and Technology (GIST) under Mi Sun Jin in 2016.
ksb1201@gist.ac.krFigure1:
Proposed transport mechanism of the CitS. Dimerization and transport domains are
shown in dark and pale blue, respectively. The helical hairpins, HP1 and HP2, of the transport
domain are represented as cylinders and loops in purple. Citrate is shown as an orange
diamond, and sodium ions as black spheres. In the apo state, the binding site of substrate and
ions is open toward the external environment. Binding of sodium ions prepares the protein to
interact with its substrate, which promotes the elevator-like movement of the transport domain
in either one of two protomers in the dimer or both. Dissociation of citrate and ions into the
cytoplasm resets the protein into the outward-facing apo state. The crystal structures of CitS
that have been determined by us and others22 are marked with black stars.