CORRECTION: The substrate-binding protein imposes directionality on an electrochemical sodium gradient-driven TRAP transporter (vol 106, pg 1778, 2009)

BIOCHEMISTRY Correction for “The substrate-binding protein imposes directionality on an electrochemical sodium gradient-driven TRAP transporter,” by Christopher Mulligan, Eric R. Geertsma, Emmanuele Severi, David J. Kelly, Bert Poolman, and Gavin H. Thomas, which appeared in issue 6, February 10, 2009, of Proc Natl Acad Sci USA (106:1778–1783; first published January 28, 2009; 10.1073/pnas.0809979106).The authors note that due to a printer's error, on page 1782, the equations in Fig. 5 were missing the F constant. The corrected figure and its legend appear below.Fig. 5.Download figure Open in new tab Download powerpointFig. 5.Model of Na+-dependent transport of sialic acid by SiaPQM. The Upper image (Import Cycle) shows the uptake of sialic acid (denoted as asterisk), driven by a (electro)chemical Na+ gradient (ΔμNa + FΔΨ; F, Faraday constant). After binding of sialic acid (asterisk) to SiaP, the liganded complex docks onto SiaQM. A minimum of two sodium ions (black dots) bind to the complex and drive the translocation of sialic acid across the membrane; the sodium ions are cotransported with sialic acid, after which the system relaxes back to the initial conformation. The Lower image (Export Cycle) shows the efflux of sialic acid under conditions that an excess of unliganded SiaP is available. The critical point is that efflux of sialic acid only occurs when unliganded SiaP docks onto SiaQM with bound substrate. Assuming tight coupling in the transport reaction, two or more Na+ ions will be exported together with sialic acid