Figure 1. Sketch of the different subunit stoichiometries.

(A) CD4 subunit stoichiometry: The number of envelope-CD4 receptor interactions (red dotted lines) one trimer requires for taking part in mediating cell entry. (B) Coreceptor subunit stoichiometry: The number of envelope-coreceptor interactions (blue dotted line) one trimer requires for taking part in mediating cell entry. (C) Fusion subunit stoichiometry: The number of fusion proteins (green triangles) that have to be exposed such that the trimer takes part in mediating cell entry. (D) Relative location of functions: Do two functions (in this sketch CD4 and coreceptor binding) have to be located on the same envelope protein within one trimer, for taking part in cell entry? Here one of the two coreceptors binds to one envelope protein to which no CD4 receptor is bound.