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The Ultrastructure of C1t, a Subcomponent of the First Component of Complement: An E.M. and Ultracentrifuge Study¹

From the Department of Biochemistry and the Institute of Immunology, University of Toronto, Toronto, Ontario M5S 1A8, Canada

Abstract

C1t is a 9.5 S 1 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 Å 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.

Footnotes

¹ This study was assisted by Medical Research Council of Canada grants to L.P. MT 2355 and R.H.P. MT 1361.