Functional Characterization of The HIV-1 NEF Acidic Cluster
The human immunodeficiency virus (HIV) infects over 39.5 million people worldwide (UNAIDS/WHO 2006 Epidemic Report). Unfortunately, various socioeconomic factors have hastened the epidemic spread of HIV in Africa and in third world countries where many individuals do not have access to appropriate healthcare. Vaccines have, as of yet, been unsuccessful as a preventive measure; however, new measures, including peptide inhibitors and microbicides, are currently being developed and studied for their efficacy. The potential of finding new targets for controlling or preventing infection relies heavily on developments relating to the basic science of the virus as well as the host environment. HIV and simian immunodeficiency virus (SIV) encode an accessory protein Nef proposed to promote pathogenesis based on evidence from HIV-infected longterm nonprogressors and SIV studies in non-human primates. The pathogenic potential of Nef has been studied in relation to some of its in vitro effects including: (1) the downregulation of major histocompatibility complex I (MHC-I), (2) the downregulation of CD4, (3) infectivity enhancement, and (4) Pak2 activation. These in vitro effects, with the exception of CD4 downregulation, rely in part upon Nef's acidic cluster which has been reported to affect Nef's redirection of cell surface MHC-I to the trans-Golgi network by binding to the cytosolic adapter phosphofurin acidic cluster sorting-1 (PACS-1) (19, 147). Because Nef is potentially significant to the course of disease progression, correlations between the amino acid composition of the Nef acidic cluster and Nef activity were characterized. Nef acidic cluster mutants reducing the acidic residues in the cluster from 4 to 2, 1, and 0 were developed. Phenotypes for these mutants indicate at least one or more of the four acidic cluster glutamates are involved not only in MHC-I downregulation, but also Pak2 activation and infectivity enhancement. However, because partial activity is observed with one acidic residue and full activity with only two acidic residues, these data lead to the following conclusion: Although the Nef acidic cluster contributes to a conformational integrity important for Nef-mediated MHC-I downregulation as well as other Nef activities, it is not a prototypical acidic cluster determinant involved in PACS-1-mediated trafficking.