Structural highlights
Function
[C2LLR0_PROMI] Catalyzes the exchange of L-carnitine for gamma-butyrobetaine and related betaines (By similarity).[HAMAP-Rule:MF_01049]
Publication Abstract from PubMed
Most secondary-active transporters transport their substrates using an electrochemical ion gradient. In contrast, the carnitine transporter (CaiT) is an ion-independent, l-carnitine/gamma-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.
Arginine oscillation explains Na+ independence in the substrate/product antiporter CaiT.,Kalayil S, Schulze S, Kuhlbrandt W Proc Natl Acad Sci U S A. 2013 Oct 7. PMID:24101465[1]
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
References
- ↑ Kalayil S, Schulze S, Kuhlbrandt W. Arginine oscillation explains Na+ independence in the substrate/product antiporter CaiT. Proc Natl Acad Sci U S A. 2013 Oct 7. PMID:24101465 doi:http://dx.doi.org/10.1073/pnas.1309071110