LARGEST KNOWN MONOMERIC GLOBULAR PROTEINS

The quaternary structures of three of the water-soluble surface proteins (immobilization or serotypic antigens 51A, 51B and 51D) of Paramecium aurelia stock 51 (syngen 4) were studied by examining the behavior of the reduced proteins under a variety of conditions. Molecular weight determinations by sedimentation equilibrium of the native and the reduced proteins revealed no significant differences between the two forms while intrinsic viscosities of these molecules, under conditions known to produce random coils, were 133.4 ml/g (51A), 115.2 ml/g (51B) and 123.0 ml/g (51D). These data support the conclusion that these proteins (mol. wt. 3.0·105, 2.6·105 and 2.7·105 g/mole) are composed of single polypeptides. In addition neither gel-permeation chromatography nor sedimentation-velocity analysis indicated the presence of associated small peptides and the reduced antigens produced one predominant zone upon starch-gel electrophoresis. The possible significance of the large size of these monomers is discussed. © 1969.