Two-dimensional crystallization of rabbit C-reactive protein on lipid monolayers

Two-dimensional (2D) crystals of rabbit C-reactive protein (CRP) have been obtained by protein binding on lipid monolayers at the air/water interface. Two different types of crystalline arrays of CRP were obtained, by specific binding and non-specific adsorption to the lipids. Electron crystallographic analysis of the negatively stained specimens showed that the unit cell parameters of the CRP 2D crystals formed by specific binding were a = 81 Å, b = 78 Å, γ = 118.35°, and those formed by non-specific adsorption were a = 74 Å, b = 67 Å, γ = 95.5°, both with the layer group p1. Projection maps were obtained at a resolution of 26 Å and 22 Å respectively. They showed that only the monomers of the CRP were packed in the 2D arrays and the orientations of the monomers on the lipid monolayers were different in the two types of crystals. By comparing the two projection maps, a preliminary shape of the CRP monomer has been derived. A model of the pentameric structure of the oligomeric CRP has been proposed.