| Abstract
| - Ligand-binding-induced conformational changes in theSalmonella typhimurium aspartatereceptor were studied using spin-labeling electron paramagneticresonance. Cysteine residues, introducedby site-directed mutagenesis at several positions in the aspartatereceptor periplasmic domain, were usedto attach covalently a thiol-specific spin label. The electronparamagnetic resonance spectra of theselabeled proteins were obtained in the presence and absence of theligand aspartate, and used to calculatethe distance change between spin labels. The results support amodel in which transmembrane signalingis executed by a combined movement of α helix 4 (which leads intotransmembrane domain 2) relativeto α helix 1 (connected to transmembrane domain 1), as well as acoming together of the two subunits.Ligand binding causes spin labels at position 39 and 179 (withinone subunit) to move further from eachother and spin labels at position 39 and 39‘ (between two subunits) tomove closer to each other. Bothof these changes are very smallless than 2.5 Å. No similarchanges were detected in any aspartatereceptor samples solubilized in detergent, suggesting that the membraneis required for these conformationalchanges. This is the first case of physically measuredligand-induced changes in a full-length 1−2transmembrane domain receptor, and the results suggest that very smallligand-induced movements canresult in large effects on the activity of downstreamproteins.
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