Transduction of activation signal that follows HIV-1 binding to CD4 and CD4 dimerization involves the immunoglobulin CDR3-like region in domain 1 of CD4

The role of CD4 during the human immunodeficiency virus type 1 (HIV-1) life cycle in T cells is not restricted to binding functions, HIV-1 binding to CD4 also triggers signals that lead to nuclear translocation of NF-kappa B and are important to the productive infection process, In addition to its cytoplasmic tail, in the ectodomain, the immunoglobulin (Ig) CDR3-like region of CD4 domain 1 seemed to play a role in this cascade of signals, We demonstrate in this work that the structural integrity of the CDR3-like loop is required for signal transduction. Substitutions of negatively charged residues by positively charged residues within the CDR3-like loop either inhibited NF-kappa B translocation after HIV-1 and gp120-anti-gp120 immune complexes binding to E91K,E92K mutants or induced its constitutive activation for E87K,D88K mutants, Moreover, A2.01-3B cells expressing the E91K,E92K mutant exhibited a lower HIV-1(Lai) replication, These cells, however, expressed p56(lck), demonstrated NF-kappa B translocation upon PMA stimulation, bound HIV-1(Lai) envelope glycoprotein with high affinity, and contained HIV-1 DNA 24 h after exposure to virus. E91K, E92K, and E87K,D88K mutant CD4 molecules were unable to bind a CD4 synthetic aromatically modified exocyclic, CDR3.AME-(82-89), that mimics the CDR3-like loop structure and binds to native cell surface CD4, This result together with molecular modeling studies indicates that the CDR3.AME-(82-89) analog binds to the CDR3-like loop of CD4 and strongly suggests that this region represents a site for CD4 dimerization, The negative charges on the CDR3-like loop thus appear critical for CD4-mediated signal transduction most likely related to CD4 dimer formation.