Laminin interactions with the apoproteins of acute-phase HDL: preliminary mapping of the laminin binding site on serum amyloid A

Abstract

During AA amyloidosis, the major basement membrane components, collagen type-IV (C-IV), entactin, laminin and perlecan codeposit both spatially and temporally with AA fibrils. Our previous work demonstrated that laminin, and collagen type-IV, can associate with high affinity to a mixture of mouse acute-phase serum amyloid A isoforms (apoSAA1, apoSAA2 andapoSAA3). However, laminin also bound to residual HDL from which apoSAAs were extracted. To characterize further laminin binding specificity for acute-phase HDL apolipoproteins, we have systematically isolated the acute-phase apoSAAs, apoA-I, apoA-II and the apoCs (I, II and III) by reverse phase high pressure liquid chromatography (RP-HPLC), and tested their laminin binding activities individually by ELISA. All the apoSAAs tested bound laminin saturably and with high affinity (K_d 2 nM). In addition, apoA-I also showed laminin binding activity (K_d a 4.6 nM). Specific binding for apoA-II and the apo-Cs was not detected. To localize the laminin binding site on apoSAA, we generated defined CNBr fragments of apoSAA I and apoSAA2, purified them by RP-HPLC, and tested their laminin binding activity by ELISA. A 53 residue peptide corresponding to residues 24-76 ofapoSAA2 had the highest laminin binding activity, followed by an 80 residue peptide corresponding to residues 24-103 of apoSAA1, both of which contain a 30 residue sequence that has changed little during evolution. In addition, a 7 residue peptide (residues 17-23), which is common to both apoSAA1 and apoSAA2, also had laminin binding activity. We postulate that laminin facilitates AA amyloidogenesis by sequestering apoSAA and providing a “surface” on which heparan sulfate-dependent fibrillogenic nucleation events can take place.

Key Words:

• AA amyloidogenesis
• apoSAA
• CNBr peptides
• laminin
• binding site