Mechanistic studies of the biogenesis and folding of outer membrane proteins in vitro and in vivo: what have we learned to date?
Bottom Line: Consequently, mechanistic details and structural information about membrane protein folding are now emerging at an ever increasing pace.This review summarises current knowledge of the mechanisms of outer membrane protein biogenesis and folding into lipid bilayers in vivo and in vitro and discusses the experimental techniques utilised to gain this information.The emerging knowledge is beginning to allow comparisons to be made between the folding of membrane proteins with current understanding of the mechanisms of folding of water-soluble proteins.
Affiliation: Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds LS2 9JT, UK; School of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT, UK.Show MeSH
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Mentions: More recently it has been realised that proteins fold via a collection of parallel pathways which make up a funnel-shaped energy landscape (Fig. 1) [17–20]. At the top of the funnel, the unfolded state represents a large ensemble of high-entropy conformations of the polypeptide chain. While unstructured, the polypeptide chain may be biased by weak, residual interactions which initiate folding . Indeed, an unfolded variant of the bacterial immunity protein Im7 has been studied recently under non-denaturing conditions, revealing conformational restriction in the regions of the protein sequence which ultimately form the native helices, emphasising the importance of such interactions in the initiation of folding . Similar conclusions have been drawn from other proteins and protein fragments under different denaturing conditions [23–28]. As folding progresses, the polypeptide chain undergoes many reorganisations aiding the formation of stabilising interactions between side-chains, the protein backbone and the solvent as the protein approaches the native state . The landscape view is an attractive one as it does not place restrictions on whether secondary structure must form before, or at the same time, as the tertiary structure. Additionally, the funnel-shaped landscape predicts the experimentally observed robustness of the folding process to destabilising mutations: if the final fold remains the most stable state relative to the unfolded ensemble, a mutation may block some of the pathways to the native state but alternative folding pathways can be utilised .
Affiliation: Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds LS2 9JT, UK; School of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT, UK.