Diffraction diagnosis of protein folding in gap junction connexons.
To diagnose the regular polypeptide conformation in gap junction membranes, the x-ray intensities diffracted from oriented specimens have been separated into a modulated component due to the coherently ordered portion of the channel-forming pairs of connexon hexamers and a diffuse component due to the disordered parts. The spherically averaged ordered protein diffraction, in the resolution range 15-4 A, was compared with intensity curves calculated from the Fourier transforms of proteins representative of the major tertiary structural classes. From this comparison the alpha-helical content of the ordered portion of the connexon was estimated to be approximately 60%. Calculation of cylindrically averaged patterns for oriented distributions of alpha-helical and beta-sheet proteins demonstrated that the ratio of the modulated diffracted intensity near 5 A spacing on the meridian and 10 A spacing on the equator observed from the gap junctions can be accounted for by alpha-helical segments inclined relative to the connexon axis. Model dimers of connexonlike hexamers were constructed from alpha-helix bundle proteins to correlate features in the calculated diffraction patterns with the model parameters. On the basis of these correlations, the ordered gap junction diffraction data indicate that alpha-helical segments centered at 38 A from the midplane of the gap have a mean radial location approximately 24 A from the hexamer axis, and an axial projected length of approximately 35 A. Thus, these alpha-helical segments traverse the hydrocarbon core of the lipid bilayer, as expected for the four hydrophobic sequences of the connexin molecule.
Tibbitts
T T
TT
Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02254-9110.
Caspar
D L
DL
Phillips
W C
WC
Goodenough
D A
DA
D000818:Animals; D017630:Connexins; D005583:Fourier Analysis; D007365:Intercellular Junctions / Q000378:metabolism* / Q000648:ultrastructure; D008099:Liver / Q000378:metabolism; D008565:Membrane Proteins / Q000378:metabolism / Q000648:ultrastructure*; D051379:Mice; D008958:Models, Molecular; D011487:Protein Conformation; D014961:X-Ray Diffraction / Q000379:methods
D017630:Connexins; D008565:Membrane Proteins