PDBsum entry 2i4b

Transport protein

PDB id: 2i4b

Contents

Protein chain

401 a.a. *

Ligands

BCT

Metals

_CA

Waters ×421

* Residue conservation analysis

PDB id:

2i4b

Name:

Transport protein

Title:

Crystal structure of bicarbonate transport protein cmpa from synechocystis sp. Pcc 6803 in complex with bicarbonate and calcium

Structure:

Bicarbonate transporter. Chain: a. Fragment: solute-binding domain. Engineered: yes

Source:

Synechocystis sp.. Organism_taxid: 1148. Strain: pcc 6803. Gene: cmpa. Expressed in: escherichia coli. Expression_system_taxid: 562.

Resolution:

1.35Å R-factor: 0.209 R-free: 0.222

Authors:

N.M.Koropatkin,T.J.Smith,H.B.Pakrasi

Key ref:

N.M.Koropatkin et al. (2007). The structure of a cyanobacterial bicarbonate transport protein, CmpA. J Biol Chem, 282, 2606-2614. PubMed id: 17121816 DOI: 10.1074/jbc.M610222200

Date:

21-Aug-06 Release date: 19-Dec-06

Protein chain

Q55460  (CMPA_SYNY3) -  Bicarbonate-binding protein CmpA from Synechocystis sp. (strain PCC 6803 / Kazusa)

Seq: 452 a.a.

Struc: 401 a.a.

DOI no: 10.1074/jbc.M610222200

J Biol Chem 282:2606-2614 (2007)

PubMed id: 17121816

The structure of a cyanobacterial bicarbonate transport protein, CmpA.

N.M.Koropatkin, D.W.Koppenaal, H.B.Pakrasi, T.J.Smith.

ABSTRACT

Cyanobacteria, blue-green algae, are the most abundant autotrophs in aquatic environments and form the base of the food chain by fixing carbon and nitrogen into cellular biomass. To compensate for the low selectivity of Rubisco for CO2 over O2, cyanobacteria have developed highly efficient CO2-concentrating machinery of which the ABC transport system CmpABCD from Synechocystis PCC 6803 is one component. Here, we have described the structure of the bicarbonate-binding protein CmpA in the absence and presence of bicarbonate and carbonic acid. CmpA is highly homologous to the nitrate transport protein NrtA. CmpA binds carbonic acid at the entrance to the ligand-binding pocket, whereas bicarbonate binds in nearly an identical location compared with nitrate binding to NrtA. Unexpectedly, bicarbonate binding is accompanied by a metal ion, identified as Ca2+ via inductively coupled plasma optical emission spectrometry. The binding of bicarbonate and metal appears to be highly cooperative and suggests that CmpA may co-transport bicarbonate and calcium or that calcium acts a cofactor in bicarbonate transport.

Selected figure(s)

Figure 1.
FIGURE 1. Schematic representation of the assembled CmpABCD bicarbonate transporter. CmpA is tethered to the periplasmic membrane by a flexible linker and captures bicarbonate in the periplasm for delivery to the transmembrane complex created by CmpB. In many ABC transporters, the transmembrane pore is created by a dimer of two transmembrane-spanning polypeptides. CmpC and CmpD are ATPases that couple ATP hydrolysis to bicarbonate transport through the pore. CmpC is unique in that it contains a C-terminal solute-binding domain homologous to CmpA. SBP, solute-binding protein.

Figure 2.
FIGURE 2. The structure of CmpA. a, stereo ribbon diagram of the CmpA crystal structure at pH 5.0 with carbonic acid. The protein is gradiently colored blue to red as the chain extends from the N to the C terminus. The view is of the front of the C-clamp, which opens to the ligand-binding cleft. Carbonic acid is depicted as spheres. b, stereo ribbon diagram of the CmpA crystal structure at pH 8.0 with Ca^2+ and bicarbonate, with the identical view and coloring as in a. Bicarbonate and Ca^2+ are depicted as spheres. Note the difference in the position of carbonic acid and bicarbonate in the ligand-binding cleft.

The above figures are reprinted by permission from the ASBMB: J Biol Chem (2007, 282, 2606-2614) copyright 2007.

Figures were selected by the author.