The Glycosaminoglycan Binding Site Governs Ligand Binding to the Somatomedin B Domain of Vitronectin

doi:10.1074/jbc.272.15.9971

The Glycosaminoglycan Binding Site Governs Ligand Binding to the Somatomedin B Domain of Vitronectin*

Dietmar Seiffert^‡

From the Department of Vascular Biology, The Scripps Research Institute, La Jolla, California 92037

^‡ To whom correspondence should be addressed:
E400/3438, Experimental Station, Dupont Merck Research Laboratories, P. O. Box 80400, Wilmington, DE 19809-0400.
Tel.: 302-695-7069; Fax.: 302-695-4162; E-mail: seiffeda{at}a1.lldmpc.umc.dupont.com

Abstract

The ligand binding functions of vitronectin (Vn) are regulated by its conformational state/degree of multimerization. In the native plasma form of Vn, the C-terminal glycosaminoglycan (GAG) binding domain is believed to be cryptic. Here, evidence is provided that the addition of fucoidan or dextran sulfate to unfractionated plasma results in the formation of covalently and non-covalently stabilized Vn multimers. These multimers express conformationally sensitive antibody epitopes and ligand binding sites located in the N terminus of the Vn molecule. While heparin forms complexes with monomeric plasma Vn and induces conformational changes, a reduction in ionic strength is required for induction of multimerization. In addition, heparin serves as a template for the assembly of type 1 plasminogen activator inhibitor-induced disulfide-linked Vn multimers. These results support a new model for the structure of native Vn. The C-terminal GAG binding domain is predicted to be exposed in the native conformation, whereas the N terminus is cryptic. Ligand binding to the GAG binding site unfolds the N terminus, thereby exposing cryptic ligand binding sites.

Footnotes

↵* This work was supported by California Tobacco-related Disease Research Program Grant 4KT-0192 and National Institutes of Health Grant HL50704. The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
↵¹ The abbreviations used are:

Vn

vitronectin

GAG

glycosaminoglycan

mAb

monoclonal antibody

PBS

phosphate-buffered saline

PAI-1

type 1 plasminogen activator inhibitor

PAGE

polyacrylamide gel electrophoresis

SMB

somatomedin B

PCR

polymerase chain reaction

PPP

platelet-poor plasma

ELISA

enzyme-linked immunosorbent assay

LMW

low molecular weight.
↵² D. Seiffert and J. W. Smith, submitted for publication.
- Received November 4, 1996.
- Revision received January 27, 1997.