Interaction between Nef and INI1/SMARCB1 augments replicability of HIV-1 in resting human peripheral blood mononuclear cells

Function of ubiquitin (Ub) specific protease 15 (USP15) in HIV-1 replication and viral protein degradation

HIV-1 Nef is necessary and may be sufficient for HIV-1-associated AIDS pathogenicity, in that knockout of Nef alone can protect HIV-infected patients from AIDS. We therefore investigated the feasibility of physical knockout of Nef, using the host ubiquitin proteasome system in HIV-1-infected cells. Our co-immunoprecipitation analysis demonstrated that Nef interacted with ubiquitin specific protease 15 (USP15), and that USP15, which is known to stabilize cellular proteins, degraded Nef. Nef could also cause decay of USP15, although Nef-mediated degradation of USP15 was weaker than USP15-mediated Nef degradation. Direct interaction between Nef and USP15 was essential for the observed reciprocal decay of the proteins. Further, USP15 degraded not only Nef but also HIV-1 structural protein, Gag, thereby substantially inhibiting HIV-1 replication. However, Gag did not degrade USP15, indicating that the Nef and USP15 complex, in distinction to other viral proteins, play an integral role in coordinating viral protein degradation and hence HIV-1 replication. Moreover, Nef and USP15 globally suppressed ubiquitylation of cellular proteins, indicating that these proteins are major determinants for the stability of cellular as well as viral proteins. Taken together, these data indicate that Nef and USP15 are vital in regulating degradation of viral and cellular proteins and thus HIV-1 replication, and specific degradation of viral, not cellular proteins, by USP15 points to USP15 as a candidate therapeutic agent to combat AIDS by eliminating viral proteins from the infected cells via USP15-mediated proteosomal degradation.

Comparative molecular genetic analysis of simian and human HIV-1 integrase interactor INI1/SMARCB1/SNF5

Human integrase interactor 1 (INI1/SMARCB1/SNF5) is a chromatin-remodeling molecule that binds to HIV-1 integrase and enhances proviral DNA integration. INI1 is also known as a tumor suppressor gene and has been found to be mutated in several aggressive tumors such as rhabdoid and lymphoid tumors. To study the function of simian INI1, we screened and cloned simian INI1 cDNA from B lymphoma cells of rhesus monkeys using RT-PCR. Sequence analysis showed 23 single nucleotide differences compared to the human ortholog, which, however, did not result in amino acid changes, and the amino acid sequence is therefore 100% conserved between human and simian INI1. Two alternatively spliced isoforms, INI1a and INI1b, were also found in simian INI1. These two isoforms did not show any functional difference in HIV-1 proviral DNA integration and nuclear localization, suggesting that the specificity of simian INI1 would not be a factor preventing HIV-1 infection of a simian host. Nevertheless, INI1b is expressed only in established cancer cell lines such as Jurkat and COS-7 cells, and not in primary cells, suggesting that INIlb could be an indicator of cell transformation.