Pentraxin binding to isolated rat liver nuclei

The interaction of human C-reactive protein (CRP) and serum amyloid P-component (SAP) with isolated rat liver nuclei was studied to identify nuclear ligands for each pentraxin using the iodinatable heterobifunctional thiol-cleavable crosslinking reagent sulphosuccinimidyl-2 -(p- azidosalicylamido)-1,3'-dithiopropionate (SASD). Nuclei (100 μg of DNA) bound 21 pmol of ¹²⁵I-labelled CRP Ca²⁺-dependently at saturation with half-saturation occurring at 200 pmol of ¹²⁵I-CRP. By contrast, only 2.7 pmol of ¹²⁵I-labelled SAP was bound at saturation, with half-saturation at 50 pmol. The binding of pentraxins to nuclei is, in addition to putative chromatin binding, due to nuclear-envelope binding, where 3.2 pmol ¹²⁵I-labelled CRP binds Ca²⁺ dependently to nuclear envelopes (25 μg) at saturation, but only 0.62 pmol SAP is required to saturate. Specific photocross-linking of ¹²⁵I-2-(p-azidosalicylamido)-1,3'-dithiopropionate (¹²⁵I-ASD)-CRP and ¹²⁵I-ASD-SAP to nuclei revealed transfer of ¹²⁵I-photoreactive azides to nuclear-envelope proteins of 43, 46, 52 and 70 kDa. In addition, SAP binding to histones H2A, H2B, H3 and H4 was detected, whereas CRP bound only to H4. Neither pentraxin cross-linked to histone H1.