Functional characterization of soluble and membrane-bound forms of vaccinia virus complement control protein (VCP)

Vaccinia virus secretes a 35 kD protein, vaccinia virus complement control protein (VCP), that inhibits the classical and alternative pathways of comp lement at several points, indicating that it may be a viral analogue of hum an complement receptor type 1 (CR1; CD35). Structurally, however, CR1 is co mposed of 30 short consensus repeats (SCRs), whereas VCP consists entirely of four SCRs, We have begun a structure-function analysis of VCP to define the minimum number of SCRs necessary for function, the functional differenc es between VCP and CR1, and the potential therapeutic roles for VCP. We add ressed these questions by creating and characterizing recombinant soluble a nd membrane-bound forms of VCP. We have determined that (1) VCP requires al l four SCRs to bind C3b, (2) whereas CR1 binds C3b and iC3b, VCP binds C3b but not iC3b, and (3) although normally secreted, if expressed on the membr ane of mammalian cells, VCP effectively protects the cells from complement- mediated lysis. Thus, VCP appears to be a compact and unique complement reg ulatory protein with the ability to inhibit both arms of the complement cas cade, but lacking affinity for iC3b. By releasing rather than capturing iC3 b-bearing complexes following inactivation of C3b, VCP may 'recycle' its ac tive site locally among infected cells, and thereby enable the virus to eva de more efficiently host immune and inflammatory responses. The unique func tion, compact structure, and capacity of VCP to protect mammalian cells fro m complement-mediated attack, suggests that it could be used both to better understand the structure-function relationship of complement regulatory pr oteins, in general, and also to rationally design and develop novel therape utic agents. (C) 1999 Elsevier Science Ltd. All rights reserved.