Backbone dynamics of a model membrane protein: carbon-13 NMR spectroscopy of alanine methyl groups in detergent-solubilized M13 coat protein

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Abstract
The filamentous coliphage M13 possesses multiple copies of a 50-residue coat protein which is inserted into the inner membrane of Escherichia coli during infection. 13C nuclear magnetic resonance (NMR) spectroscopy has been used to probe the structure and dynamics of M13 coat protein solubilized in detergent micelles. A comparison of backbone dynamics within the hydrophobic core region and the hydrophilic terminal domains was obtained by biosynthetic incorporation of [3-13C]alanine. Alanine is distributed throughout the protein and accounts for 10 residues (i.e., 20% of the total). Similar 13C NMR spectra of the protein have been obtained in two anionic detergents, sodium deoxycholate and sodium dodecyl sulfate, although the structures and physical properties of these solubilizing agents are quite different. The N-terminal alanine residues, assigned by pH titration, and the penultimate residue, assigned by carboxypeptidase A digestion, give rise to analogous peaks in both detergent systems. The pKa of Ala-1 (.apprx. 8.8) and the relaxation parameters of individual carbon atoms (T1, T2, and the nuclear Overhauser enhancement) are also generally similar, suggesting a similarity in the overall protein structure. Relaxation data have been analyzed according to the model-free approach of Lipari and Sazbo [Lipari, G., and Szabo, A. (1982) J. Am. Chem. Soc. 104, 4546-4559]. The overall correlation times were obtained by fitting the three experimental relaxation values for a given well-resolved single carbon atom to obtain a unique value for the generalized order parameter, S2, and the effective correlation time, .tau.e. The former parameter reflects the spatial restriction of motion, and the latter, the rate. Assuming symmetric rotation of the methyl group, S2 effectively reports the reorientation of the C2.sbd.C3 axis, i.e., protein backbone motions. The polypeptide backbone is thus shown to approach rigidity in the hydrophobic core and parts of the hydrophilic regions. Substantial mobility is apparent only at the extreme ends of the polypeptide chain (Ala-1 and Ala-49).