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Title: Light-driven Na+ pump from Gillisia limnaea: A high-affinity Na+ binding site is formed transiently in the photocycle

A group of microbial retinal proteins most closely related to the proton pump xanthorhodopsin has a novel sequence motif and a novel function. Instead of, or in addition to, proton transport, they perform light-driven sodium ion transport, as reported for one representative of this group (KR2) from Krokinobacter. In this paper, we examine a similar protein, GLR from Gillisia limnaea, expressed in Escherichia coli, which shares some properties with KR2 but transports only Na+. The absorption spectrum of GLR is insensitive to Na+ at concentrations of ≤3 M. However, very low concentrations of Na+ cause profound differences in the decay and rise time of photocycle intermediates, consistent with a switch from a “Na+-independent” to a “Na+-dependent” photocycle (or photocycle branch) at ~60 μM Na+. The rates of photocycle steps in the latter, but not the former, are linearly dependent on Na+ concentration. This suggests that a high-affinity Na+ binding site is created transiently after photoexcitation, and entry of Na+ from the bulk to this site redirects the course of events in the remainder of the cycle. A greater concentration of Na+ is needed for switching the reaction path at lower pH. The data suggest therefore competition between H+ and Na+more » to determine the two alternative pathways. The idea that a Na+ binding site can be created at the Schiff base counterion is supported by the finding that upon perturbation of this region in the D251E mutant, Na+ binds without photoexcitation. Furthermore, binding of Na+ to the mutant shifts the chromophore maximum to the red like that of H+, which occurs in the photocycle of the wild type.« less
Authors:
 [1] ;  [1] ;  [1] ;  [1] ;  [2] ;  [1]
  1. Univ. of California, Irvine, CA (United States)
  2. Sogang Univ., Seoul (Korea)
Publication Date:
OSTI Identifier:
1165228
Grant/Contract Number:
FG03-86ER13525; DEFG03-86ER13525
Type:
Published Article
Journal Name:
Biochemistry
Additional Journal Information:
Journal Volume: 53; Journal Issue: 48; Journal ID: ISSN 0006-2960
Publisher:
American Chemical Society (ACS)
Research Org:
Univ. of California, Irvine, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; 59 BASIC BIOLOGICAL SCIENCES; 60 APPLIED LIFE SCIENCES