TRIM5 proteins mediate a potent block to the cross-species transmission of retroviruses, the most well known being the TRIM5␣ protein from rhesus macaques, which potently inhibits human immunodeficiency virus type 1 (HIV-1) infection. This restriction occurs at an early stage in the replication cycle and is mediated by the binding of TRIM5 proteins to determinants present in the retroviral capsid. TRIM5␣, as well as other TRIM family proteins, has been shown to be regulated by interferons (IFN). Here we show that TRIM5␣ associates with another IFN-induced gene, sequestosome-1/p62 (p62). p62 plays a role in several signal transduction cascades that are important for maintaining the antiviral state of cells. Here we demonstrate that p62 localizes to both human and rhesus macaque TRIM5␣ cytoplasmic bodies, and fluorescence resonance energy transfer (FRET) analysis demonstrates that these proteins closely associate in these structures. When p62 expression was knocked down via small interfering RNA (siRNA), the number of TRIM5␣ cytoplasmic bodies and the level of TRIM5␣ protein expression were reduced in cell lines stably expressing epitope-tagged versions of TRIM5␣. In accordance with these data, p62 knockdown resulted in reduced TRIM5␣-mediated retroviral restriction in cells expressing epitope-tagged TRIM5␣ or expressing endogenously expressed human TRIM5␣. p62 may therefore operate to enhance TRIM5␣-mediated retroviral restriction, contributing to the antiviral state of cells following IFN treatment.

The conversion of bovine Factor V (FV) to Factor Va (FVa) by thrombin appears to proceed in two steps as shown below. FV(300,000) → thrombin Factor V intermediate (FVi) 2 sub-units; 220,000 & 100,000)→FVa (2 subunits; FVa-H.C., 100,000 & FVa-L.C., 73,000)+activation peptide/s. The 220,000 Mol. wt. subunit of FVi gives rise to the FVa-L.C. EDTA dissociates both FVi and FVa into subunits, Mn2+ facilitates reassociation. Even following dissociation with EDTA, 1 mole of Ca2+ remains bound to each of the FVa subunits.Support of the above model of FV activation comes from an immunological characterization of the FV. Antibodies to FV cross react with FVa, FVi and both subunits of Fva. Antibodies to FVa cross react with FVi both subunits of FVi and both subunits of FVa. Antibodies to the FVa-L.C. cross react with FV, FVj and FVa, but do not with the FVa-H.L. Antibodies to the FVa-H.C. cross react with FV, the FVi, FVa but not with the FVa-L. C.A protease from Russell’s viper venom activated FV by making a single proteolytic cleavage. This results in the formation of two subunits of Mol. wt., 250,000 & 73,000. The 73,000 Mol. wt. subunit is functionally and immunologically identical to the FVa-L.C. the 250,000 Mol. wt. subunit contains the antigenic determinants to the FVa-H.C. and can be converted into this subunit by incubation with thrombin. The appearance of two disdinct high Mol. wt. activation products from FV; one containing the FVa-L.C.; the other containing the FVa-H.C., is compatible with placing the activation peptide region between the two subunits of Fva.