The ultrastructure of C1t, a subcomponent of the first component of complement: an E.M. and ultracentrifuge study

C1t is a 9.5 S alpha1 glycoprotein that has been shown to be a fourth subcomponent of the first component of complement. The m.w. of C1t was found to be 233,000 by sedimentation equilibrium in the ultracentrifuge. A subunit m.w. of 23,000 was obtained by sedimentation equilibrium in 5.95 M guanidinium chloride. No change in either m.w. was produced by prior reduction and alkylation. In the electron microscope characteristic pentagonal figures of 85 A diameter were observed together with rod-like figures which appear to be stacked assemblies of the pentagonal figures. These observations lead us to propose that C1t is a noncovalent, decameric protein with the subunits disposed at the vertices of two regular pentagons joined at one of their faces. A possible relationship between C1t and the P-component of amyloid is discussed.