Tetrameric assembly of full-sequence protein zero myelin glycoprotein by synchrotron X-ray scattering

Highly purified myelin PO glycoprotein was solubilized to 1-8 mg/ml in 0.1% sodium dodecyl sulfate (SDS), and the solution structure of the PO assembl y was studied using synchrotron x-ray scattering. The full-length PO, which was isolated from bovine intradural roots, included both the extracellular and cytoplasmic domains of the molecule. At the higher concentrations (4, 6, and 8 mg/ml, respectively), an x-ray intensity maximum was observed at 3 76 Angstrom, 245 Angstrom, and 240 Angstrom Bragg spacing. Because the posi tion of this intensity depended on PO concentration, it is most likely due to interparticle interference. By contrast, the position of a second intens ity maximum, which was at similar to 40 Angstrom Bragg spacing, was invaria nt with PO concentration. This latter intensity was accounted for by monodi spersed, 80 Angstrom-diameter particles that are composed of eight, similar to 30 Angstrom-diameter spheres. Chemical parameters suggest that the 80 A ngstrom particles correspond to the size of a tetramer of PO molecules. The refore, the similar to 30 Angstrom spheres would correspond to the sizes of the extracellular and cytoplasmic domains for each of the PO monomers. The invariance of the second intensity maximum with PO concentration indicates that the structure of the 80 Angstrom-diameter, tetrameric particles is un altered. According to the liquid model for interparticle interference from charged spheres, the 80 Angstrom-diameter particle has 10 negative surface charges which likely arise from negatively charged SDS molecules bound to t he transmembrane domain of PO. This binding, however, apparently does not a lter the tetrameric assembly of PO, suggesting that intermolecular interact ions involving extracellular domains and cytoplasmic domains likely stabili ze this assembly. Some of our results have been published in abstract form (Inouye, H., H. Tsuruta, D. A. Kirschner, J. Sedzik, and K. Uyemura. Abstra cts of the 4th International School and Symposium on Synchrotron Radiation in Natural Science, June 15-20, 1998. Ustron-Jaszowiec, Poland. p. 31).