The targeting of CD4 to non-raft domains of the cell membrane can block HIV-1 infection, according to work from del Real and colleagues published in The Journal of Experimental Medicine.

Rafts (cholesterol-enriched lipid domains in the plasma membrane) are known to be important for regulating both cell signalling and pathogen infection of cells. The HIV-1 protein gp120 binds to CD4, which forms a complex with a chemokine receptor (CCR5 or CXCR4); HIV-1 needs this receptor complex to infect target cells. Although the role of rafts in CD4-mediated signalling is well established, it is not known whether the association of CD4 with rafts is important for the entry of HIV-1 into cells.

To answer this question, the authors generated CD4 chimaeras and mutants (in which the transmembrane and cytoplasmic domains were affected) and investigated the effect of these mutations on the partitioning of CD4 to raft or non-raft domains, CD4 signalling and HIV-1 infection. Of the mutants, only CD4-LDL — generated by replacing the CD4 transmembrane and cytoplasmic domains with those from the low-density lipoprotein (LDL) receptor — partitioned to the non-raft plasma-membrane fraction in transfected cells; all of the other mutants retained raft partitioning. Furthermore, CD4-LDL failed to activate the tyrosine kinase LCK after CD4 crosslinking and also failed to mediate HIV-1 entry (although it bound as well as wildtype CD4 to gp120). All mutants that retained raft partitioning mediated both CD4-induced LCK activation and HIV-1 entry.

Why is CD4 raft partitioning required for HIV-1 entry? Confocal microscopy studies showed that cells transfected with wildtype CD4 formed raft-associated complexes of CD4, gp120 and CXCR4 after gp120 induction. However, in CD4-LDL-transfected cells, CD4 and gp120 co-localized in non-raft membranes, but these did not coalesce with CXCR4-containing rafts. So, the partitioning of CD4 to rafts is required for gp120-induced receptor clustering.

These results show that the partitioning of CD4 to rafts is required for HIV-1 entry, and they establish that HIV-1 exploits rafts as host-cell entry sites, which might enable new strategies for preventing HIV-1 infection to be developed.