STRUCTURE SOLUTION OF SOME C-REACTIVE PROTEINS. Annette K. Shrive, David Holden, Allison Metcalfe, Dean A.A. Myles, Margaret Hopkins, Ian D. Glover, David Hoole¹, Anne C. Bloomer² and Trevor J. Greenhough^*, Dept. of Physics, and ¹Biological Sciences, Keele University, Keele, Staffs, ST5 5BG, UK. ²MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK. * and CCLRC Daresbury Laboratory, Warrington WA4 4AD, UK

C-reactive protein (CRP) is a member of the pentraxin family, a conserved, phylogenetically ancient super-family of oligomeric calcium-binding proteins exhibiting high sequence homologies, often forming a cyclic pentameric assembly. Studies centred at Keele have produced the structure of Ca-free rat CRP and Ca-bound human CRP, with the structure solution of Ca-free human CRP and CRP from Limulus polyphemus in progress.

Human CRP is a trace plasma protein of unknown biological function that is expressed dramatically as part of the acute-phase response and as such is used almost universally as a clinical test for infection and inflammation. The generation of a pentameric model from the pentraxin serum amyloid P (SAP) has finally provided the structure solution for both the rat and human crystals. The Ca-bound human structure contains a crystal contact where the calcium-binding loop from one protomer coordinates into the calcium site of a protomer in a symmetry related pentamer, revealing the mode of binding of the principal ligand phosphocholine (PC). A striking structural cleft, on the pentameric face opposite to the PC binding site, suggests an important functional role, perhaps in complement activation.

Rat CRP, in contrast to human CRP with which it shares 65% sequence identity, is a normal component of serum, is glycosylated, has been reported to contain a disulphide-linked dimer, and exhibits comparable affinities for phosphoethanolamine (PE) and PC. Native data to 3.3Å, from a single cryocooled small crystal, and the rebuilt pentameric SAP model have provided the structure solution. Conformational changes with respect to human SAP seen in human CRP are also present in rat CRP. The proposed disulphide linkages and PC/PE binding are discussed.