A stable folding core in the folding transition state of an alpha helical integral membrane protein

P Curnow, N.D Di Bartolo, K.M Moreton, O.O Ajoje, N.P Saggese, P.J Booth

Research output: Contribution to journalArticle (Academic Journal)peer-review

38 Citations (Scopus)

Abstract

Defining the structural features of a transition state is important in understanding a folding reaction. Here, we use Φ-value and double mutant analyses to probe the folding transition state of the membrane protein bacteriorhodopsin. We focus on the final C-terminal helix, helix G, of this seven transmembrane helical protein. Φ-values could be derived for 12 amino acid residues in helix G, most of which have low or intermediate values, suggesting that native structure is disrupted at these amino acid positions in the transition state. Notably, a cluster of residues between E204 and M209 all have Φ-values close to zero. Disruption of helix G is further confirmed by a low Φ-value of 0.2 between residues T170 on helix F and S226 on helix G, suggesting the absence of a native hydrogen bond between helices F and G. Φ-values for paired mutations involved in four interhelical hydrogen bonds revealed that all but one of these bonds is absent in the transition state. The unstructured helix G contrasts with Φ-values along helix B that are generally high, implying native structure in helix B in the transition state. Thus helix B seems to constitute part of a stable folding nucleus while the consolidation of helix G is a relatively late folding event. Polarization of secondary structure correlates with sequence position, with a structured helix B near the N terminus contrasting with an unstructured C-terminal helix G.
Translated title of the contributionA stable folding core in the folding transition state of an alpha helical integral membrane protein
Original languageEnglish
Pages (from-to)14133 - 14138
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume108
Issue number34
DOIs
Publication statusPublished - Aug 2011

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