Polymorphism of structural forms of C-reactive protein.

C-reactive protein (CRP), as a classical member of acute phase reactants, plays a variety of physiological functions in vivo and in vitro. Here we show that CRP exists in different structural forms in solution and on phospholipid membranes. At least three types of CRP structures, i.e., the pentameric ring-like form, the small globulin-like form and the fibril-like structure, were observed by combination of size-exclusion chromatography and electron microscopy. The pentameric ring-like CRPs were discovered mostly on ligand containing membrane in a calcium-dependent manner. The globulin-like monomers found on negatively charged membrane in the absence of calcium exhibited structural stability. The fibril-like structures, that have not been reported before, were formed by face-to-face stacking of pentameric CRP in a number from several to hundreds. The freshly purified CRPs form short single-strand fibrils while those CRPs stored for more than several days form long and bundled fibrils. The different structural forms may convert to each other under certain conditions, which gives some hints as to the structural basis of multiple functions of CRP.