Bone marrow stromal antigen 2 (BST-2, also known as tetherin) is a recently identified interferon-inducible host restriction factor that can block the production of enveloped viruses by trapping virus particles at the cell surface. This antiviral effect is counteracted by the human immunodeficiency virus type 1 (HIV-1) accessory protein viral protein U (Vpu). Here we show that HIV-1 Vpu physically interacts with BST-2 through their mutual transmembrane domains and leads to the degradation of this host factor via a lysosomal, not proteasomal, pathway. The degradation is partially controlled by a cellular protein, ␤-transducin repeat-containing protein (␤TrCP), which is known to be required for the Vpu-induced degradation of CD4. Importantly, targeting of BST-2 by Vpu occurs at the plasma membrane followed by the active internalization of this host protein by Vpu independently of constitutive endocytosis. Thus, the primary site of action of Vpu is the plasma membrane, where Vpu targets and internalizes cell-surface BST-2 through transmembrane interactions, leading to lysosomal degradation, partially in a ␤TrCP-dependent manner. Also, we propose the following configuration of BST-2 in tethering virions to the cell surface; each of the dimerized BST-2 molecules acts as a bridge between viral and cell membranes. Viral protein U (Vpu)2 is an 81-amino acid type I integral membrane phosphoprotein expressed by human immunodeficiency virus type 1 (HIV-1) (1, 2) and several simian immunodeficiency viruses (3-6). Vpu is not incorporated into virus particles (7), indicating that it acts exclusively in virus-producer cells. Indeed, Vpu is known to play two distinct roles during the later stages of infection. First, Vpu interacts with newly synthesized CD4 molecules complexed with the gp160 envelope glycoprotein precursor in the endoplasmic reticulum (8, 9) and recruits the ␤-transducin repeat-containing protein 1 (␤TrCP-1) subunit of the Skp1-Cullin1-F-box ubiquitin ligase complex (10) as well as ␤TrCP-2 (11) through its phosphoserine residues at positions 52 and 56 in the cytoplasmic (CT) domain (12,13). This event results in proteasome-mediated degradation of CD4 (10, 14, 15) allowing gp160 to resume transport toward the cell surface for virion incorporation. Second, Vpu mediates the enhancement of virion release (16 -18) in a cell type-dependent manner (e.g. HeLa cells require Vpu, whereas COS7 cells do not (19,20)), and its absence leads to the accumulation of viral particles at the cell surface (21).In contrast to the effect of Vpu on CD4 degradation, little had been known about the mechanism by which Vpu enhances the release of virions. The finding that HeLa-COS7 heterokaryons exhibited HeLa-type properties suggested that Vpu-responsive HeLa cells might harbor endogenous a restriction factor(s) that could be counteracted by this viral protein (22), as seen in Vifresponsive cells harboring the antiretroviral factor APOBEC3G counteracted by Vif (23). Neil et al. (24) showed that Vpu-deficient viral particles accumulated ...

Five actively secreted proteins (MPT32, MPT45, MPT51, MPT53, and MPT63) and the MPT46 protein were purified to homogeneity from Mycobacterium tuberculosis culture fluid and compared with proteins previously purified by ourselves and other investigators. Antisera were obtained by immunization of rabbits with all of the newly isolated proteins identified to be immunogenic. Two-dimensional electrophoresis of culture fluids obtained each week for 2 to 10 weeks of culturing of M. tuberculosis revealed characteristic changes, permitting identification of two distinct groups of proteins being actively secreted from the mycobacterial cells or appearing later in the culture fluids as a result of the release of soluble proteins from the cytosol after lysis of bacteria. The N-terminal amino acid sequences of five MPTs were shown to be identical to those of proteins previously isolated by other investigators and given different designations, and five new sequences are given. These sequences and the use of the antisera may serve to identify these proteins with mycobacterial constituents isolated by other investigators. The previously identified but not isolated MPT45 protein was shown to correspond to the C component of the antigen 85 complex. The 27-kDa MPT51 protein was demonstrated to cross-react with the three components of the antigen 85 complex, and the N-terminal amino acid sequences of MPT51 and MPT59 showed 60% homology. This finding and the extensive cross-reactivity between the components of the antigen 85 complex may indicate that there is a family of closely related secreted proteins in mycobacteria.

Approximately 17% of the human genome is comprised of long interspersed nuclear element 1 (LINE-1, L1) non-LTR retrotransposons. L1 retrotransposition is known to be the cause of several genetic diseases, such as hemophilia A, Duchene muscular dystrophy, and so on. The L1 retroelements are also able to cause colon cancer, suggesting that L1 transposition could occur not only in germ cells, but also in somatic cells if innate immunity would not function appropriately. The mechanisms of L1 transposition restriction in the normal cells, however, are not fully defined. We here show that antiretroviral innate proteins, human APOBEC3 (hA3) family members, from hA3A to hA3H, differentially reduce the level of L1 retrotransposition that does not correlate either with antiviral activity against Vif-deficient HIV-1 and murine leukemia virus, or with patterns of subcellular localization. Importantly, hA3G protein inhibits L1 retrotransposition, in striking contrast to the recent reports. Inhibitory effect of hA3 family members on L1 transposition might not be due to deaminase activity, but due to novel mechanism(s). Thus, we conclude that all hA3 proteins act to differentially suppress uncontrolled transposition of L1 elements.

Antiretroviral cytidine deaminase APOBEC3G, which is abundantly expressed in peripheral blood lymphocytes and macrophages, strongly protects these cells against HIV-1 infection. The HIV-1 Vif protein overcomes this antiviral effect by enhancing proteasome-mediated APOBEC3G degradation and is key for maintaining viral infectivity. The 579-bp-long vif gene displays high genetic diversity among HIV-1 subtypes. Therefore, it is intriguing to address whether Vif proteins derived from different subtypes differ in their viral defense activity against APOBEC3G. Expression plasmids encoding Vif proteins derived from subtypes A, B, C, CRF01_AE, and CRF02_AG isolates were created, and their anti-APOBEC3G activities were compared. Viruses produced from cells expressing APOBEC3G and Vif proteins from different subtypes showed relatively different viral infectivities. Notably, subtype C-derived Vif proteins tested had the highest activity against APOBEC3G that was ascribed to its increased binding activity, for which the N-terminal domain of the Vif protein sequences was responsible. These results suggest that the biological differences of Vif proteins belonging to different subtypes might affect viral fitness and quasispecies in vivo.Among the seven human APOBEC3 3 cytidine deaminase proteins (from A to H) that act as intrinsic restriction factors against endogenous and exogenous retroviruses (1-9), APOBEC3G provides the most potent retroviral restriction in vitro and in vivo (10 -12). This host protein is abundantly expressed in peripheral blood mononuclear cells (PBMCs) and macrophages. APOBEC3G deaminates deoxycytidine to deoxyuridine in nascent viral minus-strand cDNA, thereby inducing G-to-A hypermutations during reverse transcription (13-16). It also partially restricts viral replication in a deamination-independent fashion, mainly by blocking DNA synthesis (17)(18)(19)(20). Human immunodeficiency virus type 1 (HIV-1) is armed with Vif 3 protein, which induces proteasome-mediated APOBEC3G degradation (21-24) via a mechanism involving the Cullin5 (Cul5)-containing E3 ubiquitin ligase (25-28). As a result, Vif protein reduces virion incorporation of APOBEC3G in virus-producer cells (29 -32), leading to efficient reverse transcription in the target cells. By the same mechanism, Vif protein can inactivate APOBEC3F and APOBEC3DE, which are expressed in PBMCs and suppress Vif-deficient HIV-1 infection to a lesser extent than does APOBEC3G (33-35).Several in vivo studies have demonstrated that APOBEC3G-induced G-to-A hypermutation is frequently observed in patient-derived proviral DNA (36 -42) even in the presence of full-length but polymorphic vif genes (43). The vif genes also have high in vivo genetic variability (11, 44 -46) and subtypedependent amino acid substitutions (47). These findings imply that the sequence diversity of vif genes (possibly in a subtypedependent manner) might harbor differential levels of anti-APOBEC3G activity. Among the strains tested in the present study, Vif protein derived from subtype C strain...