Despite the fact that the early components of the mouse and human complement cascades are very similar, there are marked differences between the two species in the structure of C3 receptors and the molecules that control homologous lysis. Here, Michael Holers, Taroh Kinoshita and Hector Molina compare and contrast the mouse and human RCA region products and conclude that the receptor and regulatory roles are conserved despite the structural variation.

One of the primary roles of the classical and alternative Proteins that interact with membrane-bound C3 fall complement activation pathways is to covalently attach into two functional groups that, in general, exhibit either the complement component C3 to targets in a form receptor or non-receptor activities (reviewed in Refs known as C3b (reviewed in Ref. 1). The bound C3b 2-4). Proteins that interact with C3b and exhibit nonundergoes one of several possible fates: it may form part receptor activities serve primarily to decrease further of the classical pathway C5 convertase, or the alternative complement activation and deposition on the mempathway C3 convertase, and then continue with the rest branes in which they reside, by disrupting the formation of the activation pathway that leads to formation of the of the C3 and C5 convertases. This activity is necessary membrane attack complex; it may bind to one of its because a covalent deposition of C3 is, in part, indisreceptors and mediate clearance of its bound target; or it criminate. If unchecked, deposition of high levels of may undergo a series of site-specific proteolytic cleavage complement on self membranes can occur, possibly reactions which generate structural forms of the molecule damaging autologous tissue (homologous lysis). designated iC3b, C3dg and C3d. While the latter forms Proteins that interact with C3 (and usually C4) in this of C3 cannot participate aspart of the convertaseenzyme fashion stem from the members of the regulators of complexes which cleave C5 or more C3, they can interact complement activation (RCA) gene family 2. The best with C3 receptors and other C3-binding proteins, understood proteins of the RCA family have been defined