The Complement Cofactor Protein (SBP1) from the Barred Sand Bass (Paralabrax nebulifer) Mediates Overlapping Regulatory Activities of Both Human C4b Binding Protein and Factor H

Abstract

We have previously shown that serum of the teleost fish barred sand bass (Paralabrax nebulifer) cleaves the ␣-chain of human C4b and C3b. The proteins that participate in these reactions were purified, and a specific protease and a single cofactor protein were identified. Functional characterization of the recombinantly expressed sand bass cofactor protein (SBP1) and truncated forms containing short consensus repeats (SCRs) 1-2, 1-3, 1-4, 1-5, and 12-17 revealed that SBP1 and SCRs 1-4 mediate the functional activities of the human plasma regulatory protein C4bp and factor H. They form a complex with C4b, inhibit the formation, and accelerate the decay of the classical pathway C3 convertase and display cofactor activity for the cleavage of C4b. In contrast, the interaction of SBP1 and SCRs 1-4 with human C3b in all these activities was limited. This difference is due to species-specific incompatibilities between the cofactor protein and human C3b. SBP1 and SCRs 1-5 displayed full binding and cofactor activity for methylamine-treated C3 from trout, a species closely related to the sand bass. The presence of only one cofactor in the fish plasma that combines the functional activities of C4bp and factor H demonstrates that the sand bass cofactor protein is the ancestral precursor to the two complement regulatory proteins in human plasma.In vertebrates, cleavage of the third component of complement (C3) 1 and subsequent assembly of the membrane attack complex (C5-C9) occur as functions of at least two distinct enzymes generated during the activation of the classical (1) and alternative (2, 3) pathways. The classical pathway C3 convertase is formed by a reaction involving the reversible binding of C2 to C4b in the presence of Mg 2ϩ to form the C4b2 complex and by the cleavage of the bound C2 by C1s to generate C4b2a (4). These reactions are remarkably similar to those occurring during the formation of the alternative pathway C3 convertase (2, 3). C3b binds to factor B, which is then cleaved by factor D to generate C3bBb. The formation and function of these two enzymes are regulated by three distinct mechanisms as follows: a temperature-dependent intrinsic decay of the enzymes; an extrinsic decay mediated by the effect of the serum proteins C4-binding protein (C4bp) and factor H, and the proteolytic inactivation of the ␣Ј-chain of C4b and C3b by factor I.

A CD46-like Molecule Functional in Teleost Fish Represents an Ancestral Form of Membrane-Bound Regulators of Complement Activation

“…In barred sand bass, a factor H-like soluble RCA has been reported to show a cofactor activity in factor I-mediated degradation of both human C3b and C4b (30). It would be interesting to analyze whether Tecrem can regulate both C3b and C4b, and, if yes, which modules are responsible for the regulation, to gain evolutionary implication of this protein family through vertebrate lineage.…”

“…As for function of RCA in the lower vertebrates, regulatory roles in deposition of C3 and degradation of C3b/C4b have been described only for secreted RCAs in lamprey (35) and barred sand bass (28)(29)(30). No functional data, in contrast, have been reported for membrane-bound RCA protein in any lower vertebrate species.…”

“…Loci of each group form a compact cluster, and, in the human genome, the group 1 cluster and the group 2 loci are also located in proximity, forming a RCA gene cluster (20)(21)(22). Genomic arrangement of the RCA genes and their clustering are diverse among species analyzed to date, such as mice (23,24), chicken (25,26), frog (27), and teleost, which is the most common group of the ray-finned bony fish (28)(29)(30)(31)(32)(33)(34).…”

A Short Consensus Repeat-Containing Complement Regulatory Protein of Lamprey That Participates in Cleavage of Lamprey Complement 3

“…Sand bass SBP1 is known to be a structural homologue of human factor H (10,11). SBP1 consists of 17 tandem SCRs and binds to both rainbow trout C3b and human C4b (11), serving as cofactor for factor I (11). In its structure and function, SBP1 resembles human factor H, which possesses the ability to inactivate C4b as well as C3b (12).…”

“…A nonmammalian soluble-form of C regulator named sand bass protein 1 (SBP1) has been cloned from a bony fish, Parablax nebulifer (sand bass) (10), but no such regulator has been identified to date in invertebrates. Sand bass SBP1 is known to be a structural homologue of human factor H (10,11). SBP1 consists of 17 tandem SCRs and binds to both rainbow trout C3b and human C4b (11), serving as cofactor for factor I (11).…”