CRYSTALLOGRAPHIC STUDIES OF MAMMALIAN PENTRAXINS

D. Thompson1, M.B. Pepys2 & S.P. Wood1.

1Department of Biochemistry, School of Biological Sciences, University of Southampton, Bassett Crescent East, Southampton, SO16 7PX.
2Immunological Medicine Unit, ICMS Hammersmith Hospital, Du Cane Road, London, W12 0NN

The structure of the classic acute phase reactant human C-reactive protein (CRP), with and without its physiological ligand bound, has been solved to a resolution of 2.5 A. CRP is a normal plasma protein made up of five identical subunits to form a disc-like ring. The ligand, phosphocholine1, is often found in the membrane of cells and this crystal structure provides evidence on how CRP attaches to membranes.

Several crystal forms of human CRP have been grown and data were collected from each of these forms. Initial phases were obtained by molecular replacement using the other major pentraxin found in the human plasma, Serum Amyloid P Component (SAP)2, as the search model. The crystal structure of SAP was reported in 1994 and shares 52% sequence homology with CRP. The protomer structures of the two proteins are very similar, however the orientation of the protomers with respect to the 5-fold axes of the two proteins are different and are related by 22o. The higher affinity of CRP for Ca++ is explained by the larger number of ligands to the two Ca++ ions.

One molecule of phophocholine was found bound to each of the five subunits and all occur on one of the pentameric faces. The major interactions between CRP and phosphocholine occur at either end of the phosphocholine molecule and involve an interaction between the calcium ions and two of the phosphate oxygens at one end and an interaction between the positively charged quaternary nitrogen of phosphocholine and Glu 81 at the other end. . The unsuspected presence of a hydrophobic pocket adjacent to bound PC invites designable modification of inhibitors that may have therapeutic relevance to the newly recognised role of CRP in atherothrombotic events3. There is also a cleft present in each subunit on the opposite pentameric face and may take part in the binding to the complement protein C1q and suggests a model for interaction of CRP with damaged cell membranes leading binding of C1q and clinically relevant complement activation.

Pentraxins from hamster and mouse have also been crystallized and initial diffraction data collected but as of yet have not been fully characterised.

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  2. Pepys MB, Dash AC, Fletcher TC, Richardson N, Munn EA, Feinstein A. Analogues in other mammals and in fish of human plasma proteins C-reactive protein and amyloid P component. Nature 1978;273:168-170.
  3. Haverkate F, Thompson SG, Pyke SDM, Gallimore JR, Pepys MB. Production of C-reactive protein and risk of coronary events in stable and unstable angina. Lancet 1997;349:462-466.