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Mapping conformational changes of a type IIb Na+/Pi cotransporter by voltage clamp fluorometry.


Virkki, L V; Murer, H; Forster, I C (2006). Mapping conformational changes of a type IIb Na+/Pi cotransporter by voltage clamp fluorometry. Journal of Biological Chemistry, 281(39):28837-28849.

Abstract

The fluorescence of a fluorophore depends on its environment, and if attached to a protein it may report on conformational changes. We have combined two-electrode voltage clamp with simultaneous fluorescence measurements to detect conformational changes in a type IIb Na(+)/P(i) cotransporter expressed in Xenopus oocytes. Four novel Cys labelled with a fluorescent probe yielded voltage-and substrate-dependent changes in fluorescence (F). Neither Cys-substitution nor labelling significantly altered the mutants' electrogenic properties. Different F responses to voltage and substrate were recorded at the four sites. S155C, located in an intracellular re-entrant loop in the first half of the protein, and E451C, located in an extracellular re-entrant loop in the second half of the protein both showed Na(+), Li(+)- and P(i)-dependent F signals. S226C and Q319C, located at opposite ends of a large extracellular loop in the middle of the protein, mainly responded to changes in Na(+) and Li(+). Hyperpolarization increased F for S155C and S226C, but decreased F for Q319C and E451C. The labelling and F response of S155C, confirmed that the intracellular loop containing Ser-155 is re-entrant as it is accessible from the extracellular milieu. The behavior of S155C and E451C indicates a strong involvement of the two re-entrant loops in conformational changes during the transport cycle. Moreover, the data for S226C and Q319C suggest that also the large extracellular loop is associated with transport function. Finally, the reciprocal voltage-dependencies of the S155C-E451C and S226C-Q319C pairs suggest reciprocal conformational changes during the transport cycle for their respective local environments.

The fluorescence of a fluorophore depends on its environment, and if attached to a protein it may report on conformational changes. We have combined two-electrode voltage clamp with simultaneous fluorescence measurements to detect conformational changes in a type IIb Na(+)/P(i) cotransporter expressed in Xenopus oocytes. Four novel Cys labelled with a fluorescent probe yielded voltage-and substrate-dependent changes in fluorescence (F). Neither Cys-substitution nor labelling significantly altered the mutants' electrogenic properties. Different F responses to voltage and substrate were recorded at the four sites. S155C, located in an intracellular re-entrant loop in the first half of the protein, and E451C, located in an extracellular re-entrant loop in the second half of the protein both showed Na(+), Li(+)- and P(i)-dependent F signals. S226C and Q319C, located at opposite ends of a large extracellular loop in the middle of the protein, mainly responded to changes in Na(+) and Li(+). Hyperpolarization increased F for S155C and S226C, but decreased F for Q319C and E451C. The labelling and F response of S155C, confirmed that the intracellular loop containing Ser-155 is re-entrant as it is accessible from the extracellular milieu. The behavior of S155C and E451C indicates a strong involvement of the two re-entrant loops in conformational changes during the transport cycle. Moreover, the data for S226C and Q319C suggest that also the large extracellular loop is associated with transport function. Finally, the reciprocal voltage-dependencies of the S155C-E451C and S226C-Q319C pairs suggest reciprocal conformational changes during the transport cycle for their respective local environments.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:04 Faculty of Medicine > Institute of Physiology
07 Faculty of Science > Institute of Physiology
Dewey Decimal Classification:570 Life sciences; biology
Language:English
Date:3 August 2006
Deposited On:11 Feb 2008 12:22
Last Modified:31 Aug 2016 12:24
Publisher:American Society for Biochemistry and Molecular Biology
ISSN:0021-9258
Free access at:Publisher DOI. An embargo period may apply.
Publisher DOI:10.1074/jbc.M603861200
PubMed ID:16887801
Permanent URL: http://doi.org/10.5167/uzh-1386

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