4M8J

Crystal structure of CaiT R262E bound to gamma-butyrobetaine

Summary for 4M8J

• Entry DOI: 10.2210/pdb4m8j/pdb
• Descriptor: L-carnitine/gamma-butyrobetaine antiporter, 3-CARBOXY-N,N,N-TRIMETHYLPROPAN-1-AMINIUM (3 entities in total)
• Functional Keywords: cait, leut fold, carnitine/gamma-butyrobetaine antiporter, plasma membrane, transport protein
• Biological source: Proteus mirabilis
• Cellular location: Cell membrane; Multi-pass membrane protein (By similarity): C2LLR0
• Total number of polymer chains: 1
• Total formula weight: 56431.49

Authors

Kalayil, S. (deposition date: 2013-08-13, release date: 2013-10-09, Last modification date: 2023-09-20)

Primary citation

Kalayil, S.,Schulze, S.,Kuhlbrandt, W.
Arginine oscillation explains Na+ independence in the substrate/product antiporter CaiT.
Proc.Natl.Acad.Sci.USA, 110:17296-17301, 2013

PubMed Abstract: Most secondary-active transporters transport their substrates using an electrochemical ion gradient. In contrast, the carnitine transporter (CaiT) is an ion-independent, l-carnitine/γ-butyrobetaine antiporter belonging to the betaine/carnitine/choline transporter family of secondary transporters. Recently determined crystal structures of CaiT from Escherichia coli and Proteus mirabilis revealed an inverted five-transmembrane-helix repeat similar to that in the amino acid/Na(+) symporter LeuT. The ion independence of CaiT makes it unique in this family. Here we show that mutations of arginine 262 (R262) make CaiT Na(+)-dependent. The transport activity of R262 mutants increased by 30-40% in the presence of a membrane potential, indicating substrate/Na(+) cotransport. Structural and biochemical characterization revealed that R262 plays a crucial role in substrate binding by stabilizing the partly unwound TM1' helix. Modeling CaiT from P. mirabilis in the outward-open and closed states on the corresponding structures of the related symporter BetP reveals alternating orientations of the buried R262 sidechain, which mimic sodium binding and unbinding in the Na(+)-coupled substrate symporters. We propose that a similar mechanism is operative in other Na(+)/H(+)-independent transporters, in which a positively charged amino acid replaces the cotransported cation. The oscillation of the R262 sidechain in CaiT indicates how a positive charge triggers the change between outward-open and inward-open conformations as a unifying critical step in LeuT-type transporters.

PubMed: 24101465
DOI: 10.1073/pnas.1309071110

Experimental method

X-RAY DIFFRACTION (3.294 Å)