Heat-Shock Protein 90 Controls the Expression of Cell-Cycle Genes by Stabilizing Metazoan-Specific Host-Cell Factor HCFC1.

Molecular chaperones such as heat-shock proteins (HSPs) help in protein folding. Their function in the cytosol has been well studied. Notably, chaperones are also present in the nucleus, a compartment where proteins enter after completing de novo folding in the cytosol, and this raises an important question about chaperone function in the nucleus. We performed a systematic analysis of the nuclear pool of heat-shock protein 90. Three orthogonal and independent analyses led us to the core functional interactome of HSP90. Computational and biochemical analyses identify host cell factor C1 (HCFC1) as a transcriptional regulator that depends on HSP90 for its stability. HSP90 was required to maintain the expression of HCFC1-targeted cell-cycle genes. The regulatory nexus between HSP90 and the HCFC1 module identified in this study sheds light on the relevance of chaperones in the transcription of cell-cycle genes. Our study also suggests a therapeutic avenue of combining chaperone and transcription inhibitors for cancer treatment.

High capacity of the endoplasmic reticulum to prevent secretion and aggregation of amyloidogenic proteins.

Protein aggregation is associated with neurodegeneration and various other pathologies. How specific cellular environments modulate the aggregation of disease proteins is not well understood. Here, we investigated how the endoplasmic reticulum (ER) quality control system handles ß-sheet proteins that were designed de novo to form amyloid-like fibrils. While these proteins undergo toxic aggregation in the cytosol, we find that targeting them to the ER (ER-ß) strongly reduces their toxicity. ER-ß is retained within the ER in a soluble, polymeric state, despite reaching very high concentrations exceeding those of ER-resident molecular chaperones. ER-ß is not removed by ER-associated degradation (ERAD) but interferes with ERAD of other proteins. These findings demonstrate a remarkable capacity of the ER to prevent the formation of insoluble ß-aggregates and the secretion of potentially toxic protein species. Our results also suggest a generic mechanism by which proteins with exposed ß-sheet structure in the ER interfere with proteostasis.

The evolutionary capacitor HSP90 buffers the regulatory effects of mammalian endogenous retroviruses.

Understanding how genotypes are linked to phenotypes is important in biomedical and evolutionary studies. The chaperone heat-shock protein 90 (HSP90) buffers genetic variation by stabilizing proteins with variant sequences, thereby uncoupling phenotypes from genotypes. Here we report an unexpected role of HSP90 in buffering cis-regulatory variation affecting gene expression. By using the tripartite-motif-containing 28 (TRIM28; also known as KAP1)-mediated epigenetic pathway, HSP90 represses the regulatory influence of endogenous retroviruses (ERVs) on neighboring genes that are critical for mouse development. Our data based on natural variations in the mouse genome show that genes respond to HSP90 inhibition in a manner dependent on their genomic location with regard to strain-specific ERV-insertion sites. The evolutionary-capacitor function of HSP90 may thus have facilitated the exaptation of ERVs as key modifiers of gene expression and morphological diversification. Our findings add a new regulatory layer through which HSP90 uncouples phenotypic outcomes from individual genotypes.

Diverse fates of uracilated HIV-1 DNA during infection of myeloid lineage cells.

We report that a major subpopulation of monocyte-derived macrophages (MDMs) contains high levels of dUTP, which is incorporated into HIV-1 DNA during reverse transcription (U/A pairs), resulting in pre-integration restriction and post-integration mutagenesis. After entering the nucleus, uracilated viral DNA products are degraded by the uracil base excision repair (UBER) machinery with less than 1% of the uracilated DNA successfully integrating. Although uracilated proviral DNA showed few mutations, the viral genomic RNA was highly mutated, suggesting that errors occur during transcription. Viral DNA isolated from blood monocytes and alveolar macrophages (but not T cells) of drug-suppressed HIV-infected individuals also contained abundant uracils. The presence of viral uracils in short-lived monocytes suggests their recent infection through contact with virus producing cells in a tissue reservoir. These findings reveal new elements of a viral defense mechanism involving host UBER that may be relevant to the establishment and persistence of HIV-1 infection.

Small molecule inhibition of SAMHD1 dNTPase by tetramer destabilization.

SAMHD1 is a GTP-activated nonspecific dNTP triphosphohydrolase that depletes dNTP pools in resting CD4+ T cells and macrophages and effectively restricts infection by HIV-1. We have designed a nonsubstrate dUTP analogue with a methylene bridge connecting the α phosphate and 5' carbon that potently inhibits SAMHD1. Although pppCH2dU shows apparent competitive inhibition, it acts by a surprising allosteric mechanism that destabilizes active enzyme tetramer.

GTP activator and dNTP substrates of HIV-1 restriction factor SAMHD1 generate a long-lived activated state.

The HIV-1 restriction factor sterile α-motif/histidine-aspartate domain-containing protein 1 (SAMHD1) is a tetrameric protein that catalyzes the hydrolysis of all dNTPs to the deoxynucleoside and tripolyphosphate, which effectively depletes the dNTP substrates of HIV reverse transcriptase. Here, we establish that SAMHD1 is activated by GTP binding to guanine-specific activator sites (A1) as well as coactivation by substrate dNTP binding to a distinct set of nonspecific activator sites (A2). Combined activation by GTP and dNTPs results in a long-lived tetrameric form of SAMHD1 that persists for hours, even after activating nucleotides are withdrawn from the solution. These results reveal an ordered model for assembly of SAMHD1 tetramer from its inactive monomer and dimer forms, where GTP binding to the A1 sites generates dimer and dNTP binding to the A2 and catalytic sites generates active tetramer. Thus, cellular regulation of active SAMHD1 is not determined by GTP alone but instead, the levels of all dNTPs and the generation of a persistent tetramer that is not in equilibrium with free activators. The significance of the long-lived activated state is that SAMHD1 can remain active long after dNTP pools have been reduced to a level that would lead to inactivation. This property would be important in resting CD4(+) T cells, where dNTP pools are reduced to nanomolar levels to restrict infection by HIV-1.