CXCR4 heterogeneity in primary cells: possible role of ubiquitination

The chemokine receptor CXCR4 is a primary coreceptor for the HIV-1 virus. The predicted molecular weight (MW) of glycosylated CXCR4 is 45-47 kDa. However, immunoblots of whole cell lysates from human lymphocytes, monocytes, macrophages, and the Jurkat T-lymphocyte line revealed multiple MW isoforms of CXCR4. Three of the bands could be precipitated by anti-CXCR4 monoclonal antibodies (101 and 47 kDa) or coprecipitated with CD4 (62 kDa). Expression of these isoforms was enhanced by infection with a recombinant vaccinia virus encoding CXCR4. In immunoblots of two-dimensional gels, antiubiquitin antibodies reacted with the 62-kDa CXCR4 species from monocytes subsequent to coprecipitation with anti-CD4 antibodies. Culturing of monocytes and lymphocytes with lactacystin enhanced the amount of the 101-kDa CXCR4 isoform in immunoblots by three- to sevenfold. In lymphocytes, lactacystin also increased cell-surface expression of CXCR4, which correlated with enhanced fusion with HIV-1 envelope-expressing cells. Similar increases in the intensity of the 101-kDa isoform were seen after treatment with the lysosomal inhibitors monensin and ammonium chloride. Antiubiquitin antibodies reacted with multiple proteins above 62 kDa, which were precipitated with anti-CXCR4 antibodies. Our data indicate that ubiquitination may contribute to CXCR4 heterogeneity and suggest roles for proteasomes and lysosomes in the constitutive turnover of CXCR4 in primary human cells.