Investigating interactions of the pentraxins serum amyloid P component and C-reactive protein by mass spectrometry

The oligomeric state of human SAP (serum amyloid P component) in the absence and presence of known ligands has been investigated using nanoelectrospray ionization MS. At pH 8.0, in the absence of Ca²⁺, SAP has been shown to consist of pentameric and decameric forms. In the presence of physiological levels of Ca²⁺, SAP was observed to exist primarily as a pentamer, reflecting its in vivo state. dAMP was shown not only to promote decamerization, but also to lead to decamer stacking involving up to 30 monomers. A mechanism for this finding is proposed. CRP (C-reactive protein), a pentraxin closely related to SAP, exists as a pentamer in the presence or absence of Ca²⁺. Pentamers of CRP and SAP were shown to form mixed decamers in Ca²⁺-free buffer; however, in the presence of Ca²⁺, this interaction was not observed. Furthermore, no exchange of monomeric subunits was observed between the SAP and CRP oligomers, suggesting a remarkable stability of the individual pentameric complexes.