Direct in vitro binding of full-length human immunodeficiency virus type 1Nef protein to CD4 cytoplasmic domain

The Nef protein of the simian and human immunodeficiency viruses is known t o directly bind and down-regulate the CD4 receptor. Although the molecular mechanism is well understood, direct binding of Nef and CD4 is difficult to demonstrate and is believed to be of low affinity. Applying nuclear magnet ic resonance and fluorescence spectroscopy, we biophysically reevaluated th e CD4-Nef complex and found the dissociation constant to be in the submicro molar range. We conclude that additional, so far disregarded residues in th e N terminus of Nef are important for interaction with CD4.