SP-40,40, a newly identified normal human serum protein found in the SC5b-9 complex of complement and in the immune deposits in glomerulonephritis.

Abstract

We report herein the isolation and initial characterization of a novel protein, termed SP-40,40, which is present at moderate levels (35-105 gg/ml) in normal human serum.SP-40,40 is deposited in the renal glomeruli of patients with glomerulone- phritis but is not found in normal glomeruli.The protein is a heterodimeric structure of relative molecular mass 80 kD, both chains of which are of a similar size (40 kD).The amino-termi- nal sequences of both chains are unrelated to one another and possess no significant homology to any known protein se- quence.The tissue distribution of SP40,40 closely resembles that of the terminal complement components and its physicochemical properties are similar to, but distinct from, those of the S protein of complement.We have identified SP-40,40 in the SC5b-9 complex of complement and have demonstrated incor- poration of labeled SP-40,40 into this complex.These data suggest that SP-40,40 is an additional component of SC5b-9.1. Abbreviations used in this paper: C5b-9(m) or MC5b-9, membraneassembled attack complex of complement; GBM, glomerular base- ment membrane; IEF, isoelectric focusing; NHS, normal human serum; PEG, polyethylene glycol; SC5b-9, fluid-phase assembled at- tack complex of complement containing the S protein.