Variant Exons v6 and v7 Together Expand the Repertoire of Glycosaminoglycans Bound by CD44*
- From the Forschungszentrum Karlsruhe, Institute of Genetics, P. O. Box 3640, D-76021 Karlsruhe, Germany
Abstract
Isoforms of the glycoprotein CD44 are cell surface receptors for the glycosaminoglycan hyaluronate. They have been implicated in many biological processes, but their function in these is poorly understood and cannot be explained solely by hyaluronate binding. In the present work we examine the ligand binding properties of alternatively spliced CD44 variant isoforms which are functionally involved in the immune system, embryonic development, and tumor behavior. We show that these isoforms bind directly to the purified glycosaminoglycans chondroitin sulfate, heparin, and heparin sulfate, in addition to being able to bind to hyaluronate. Binding to this extended repertoire of glycosaminoglycans by CD44 depends on the inclusion of peptide sequences encoded by the alternatively spliced exons v6 and v7, and occurs both when the CD44 is solubilized from the plasma membrane and when it is expressed on intact cells. A single point mutation in the most N-terminal hyaluronate binding motif of CD44 ablates both hyaluronate and chondroitin sulfate binding, suggesting that glycosaminoglycans are bound through a common motif, and that only one of the hyaluronate binding motifs is responsible for the majority of glycosaminoglycan binding by CD44 on the cell surface. Taken together, these observations indicate that alternative splicing regulates the ligand binding specificity of CD44 and suggest that structural changes in the CD44 protein have a profound effect on the range of ligands to which this molecule can bind with potentially wide-ranging functional consequences.
Footnotes
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↵* This work was supported in part by the Deutsche Forschungsgemeinschaft (He 551/8–2) and Boehringer Ingelheim.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.
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↵‡ Supported by an European Molecular Biology Organization Long Term Fellowship and by a European Union HCM Fellowship. To whom correspondence should be addressed. Tel.: 49-7247-82-2714/7247-82-4354; Fax: 49-7247-82-3354.
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↵§ Current address: Chiba Cancer Center Research Institute, 666-2 Nitona, Chuo, Chiba, 260 Japan.
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↵1 The abbreviations used are: GAG, glycosaminoglycan; CPC, cetylpyridinium chloride; CS, chondroitin sulfate; CS-A, -B, and -C, chondroitin sulfate type A, B, and C (chondroitin-4-sulfate, dermatan sulfate and chondroitin-6-sulfate, respectively); HA, hyaluronate; HS, heparin sulfate; KS, keratan sulfate; H, heparin; FCS, fetal calf serum; PCR, polymerase chain reaction; PBS, phosphate-buffered saline.
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- Received March 17, 1997.
- Revision received October 9, 1997.
