INTRINSIC VERSUS IMPOSED CURVATURE IN CYCLICAL OLIGOMERS - THE PORTALPROTEIN OF BACTERIOPHAGE-SPP1

Large cyclical oligomers may be formed by (curvi-) linear polymerizati on of monomers until the n(th) monomer locks in with the first member of the chain. The subunits in incomplete structures exhibit a natural curvature with respect to each other which can be perturbed when the o ligomer closes cyclically. Using cryo-electron microscopy and multivar iate statistical image processing we report herein a direct structural observation of this effect. A sub-population (similar to 15%) of inco mplete oligomers was found within a sample of SPP1 bacteriophage porta l proteins embedded in vitreous ice. Whereas the curvature between adj acent subunits of the closed circular 13-fold symmetric oligomer is 27 .7 degrees, in these incomplete oligomers the angle is only 25.8 degre es, a value which almost allows for a 14-subunit cyclical arrangement. A simple model for the association of large cyclical oligomers is sug gested by our data.