Membrane Protein Folding and Assembly

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Only 75 membrane protein structures had been solved to high resolution by the close of 2003. Because alpha-helical membrane proteins account for ~30% of genome sequences, predicting from amino acid sequence the three-dimensional structures of these proteins is an important goal. Such predictions require understanding two fundamental issues: mechanisms of the biological assembly of MPs by the SecY/Sec61 (translocon) complex and the principles of the physical stability of MPs in their natural lipid bilayer milieu. The specific aims of this proposal address aspects of both issues as follows: (1) Decipher the "code" embedded in membrane protein amino acid sequences that determines whether or not a polypeptide segment is integrated into the endoplasmic reticulum membrane by the Sec61 complex as a transmembrane helix. This work will expand a first draft 'biological' hydrophobicity scale obtained using an in vitro system based upon cotranslational insertion of model proteins into microsomes. (2) Gain insights into translocon function using molecular dynamics simulations of the SecY complex from M.jannaschii, for which the crystallographic structure has been determined recently. These simulations will allow manipulation of the translocon structure in the bilayer environment as a means of learning how the translocon may open and close to release TM helices into the bilayer. (3) Establish an experiment-based interracial hydrophobicity scale for describing the interactions of polypeptide segments with the interface regions of bilayers formed from ER lipids. This is important because the translocon, presumably in concert with the lipid bilayer, distinguishes between the bilayer's hydrocarbon core and interfacial regions. (4) Clarify the physical basis for translocon-assisted insertion into membranes of (1) short TM helices (-12 amino acids) and (2) a model S4 helix voltage-sensor peptide from the KvAP voltage-gated K+ channel. The information obtained will help us understand how to predict the topolgy and structure of unusual membrane proteins, such as the CIC chloride channel.