1. ¹ HIV-Protein Interactions Laboratory, Centre for Virus Research,
2. ² Westmead Millennium Institute,
3. ³ National Centre for HIV Virology Research of Australia,
4. ⁴ University of Sydney,
5. ⁵ Boston Biomedical Research Institute and Harvard University Medical School, Watertown, Massachusetts,
6. ⁶ Protein Interactions Laboratory, National Cancer Institute, National Institutes of Health, Fredrick and
7. ⁷ Westmead Institute for Cancer Research
8. ⁸ Cellular and Molecular Pathology Research Unit, Department of Oral Pathology and Oral Medicine, Westmead Hospital Dental School, Westmead, NSW, Australia,
9. ⁹ Department of Microbiology and Immunology, Uniformed Services University of Health Sciences, Bethesda, Maryland, USA

Correspondence: Dr Garry W Lynch, Australian Red Cross Blood Service, Endeavour Operations Unit, PO Box 98, Camperdown, NSW 1450, Australia. Email: Garry_Lynch@optusnet.com.au

Received 21 June 2004; Accepted 18 October 2004; Published online 28 February 2005.

Abstract

CXCR4, the chemotactic cell receptor for SDF-1, is essential for immune trafficking and HIV infection. CXCR4 is remarkably heterogeneous and the purpose of this study was to better identify the isoforms expressed by cells and compare their structure and function. We found that cells express either a predominant isoform or multiple isoforms. These were best resolved on SDS-PAGE using sucrose-gradient-fractionated, triton-insoluble, membrane extracts. We hypothesized that glycosyl modification may underpin some of this heterogeneity and that cell isoform(s) differences may underscore CXCR4's multiple cell functions. A comparison of wild-type (WT) and dual N-linked glycosylation site, N11A/N176A, mutant CXCR4 expressed in 3T3 and HEK-293 cells served to implicate variabilities in glycosylation and oligomerization in almost half of the isoforms. Immunoprecipitation of CXCR4 revealed monomer and dimer non-glycosylated forms of 34 kDa and 68 kDa from the N11A/N176A mutant, compared with glycosylated 40 kDa and 47 kDa and 73 kDa and 80 kDa forms from WT. The functional specificity of isoform action was also implicated because, despite CEMT4 cells expressing high levels of CXCR4 and 11 different isoforms, a single 83 kDa form was found to bind gp120 for HIV-1 IIIB infection. Furthermore, comparative studies found that in contrast to SDF-1-responsive Nalm-6 cells that expressed similar levels of a single isoform, CEMT4 cells did not show a Ca⁺⁺ flux or a chemotactic response to SDF-1. Thus, CXCR4 can differ both structurally and functionally between cells, with HIV-1 infection and chemotaxis apparently mediated by different isoforms. This separation of structure and function has implications for understanding HIV-1 entry and SDF-1 responses and may indicate therapeutic possibilities.