The Heparin/Heparan Sulfate-binding Site on Apo-serum Amyloid A

Abstract

Serum amyloid A isoforms, apoSAA1 and apoSAA2, are apolipoproteins of unknown function that become ma-jor components of high density lipoprotein (HDL) dur-ing the acute phase of an inflammatory response. ApoSAA is also the precursor of inflammation-associ-ated amyloid, and there is strong evidence that the for-mation of inflammation-associated and other types of amyloid is promoted by heparan sulfate (HS). Data pre-sented herein demonstrate that both mouse and human apoSAA contain binding sites that are specific for hep-arin and HS, with no binding for the other major glyco-saminoglycans detected. Cyanogen bromide-generated peptides of mouse apoSAA1 and apoSAA2 were screened for heparin binding activity. Two peptides, an apoSAA1-derived 80-mer (residues 24 –103) and a smaller carbox-yl-terminal 27-mer peptide of apoSAA2 (residues 77– 103), were retained by a heparin column. A synthetic peptide corresponding to the CNBr-generated 27-mer also bound heparin, and by substituting or deleting one or more of its six basic residues (Arg-83, His-84, Arg-86, Lys-89, Arg-95, and Lys-102), their relative importance for heparin and HS binding was determined. The Lys-102 residue appeared to be required only for HS binding. The residues Arg-86, Lys-89, Arg-95, and Lys-102 are phy-logenetically conserved suggesting that the heparin/HS binding activity may be an important aspect of the func-tion of apoSAA. HS linked by its carboxyl groups to an Affi-Gel column or treated with carbodiimide to block its carboxyl groups lost the ability to bind apoSAA. HDL-apoSAA did not bind to heparin; however, it did bind to HS, an interaction to which apoA-I contributed. Results from binding experiments with Congo Red-Sepharose 4B columns support the conclusions of a recent struc-tural study which found that heparin binding domains have a common spatial distance of about 20 Å between their two outer basic residues. Our present work pro-vides direct evidence that apoSAA can associate with HS (and heparin) and that the occupation of its binding site by HS, and HS analogs, likely caused the previously reported increase in amyloidogenic conformation (␤-sheet) of apoSAA2 (McCubbin, W. D., Kay, C. M., Narin-drasorasak, S., and Kisilevsky, R. (1988) Biochem. J. 256, 775–783) and their amyloid-suppressing effects in vivo (Kisilevsky, R., Lemieux, L. J., Fraser, P. E., Kong, X., Hultin, P. G., and Szarek, W. A. (1995) Nat. Med.