Interaction of transportin-SR2 with Ras-related nuclear protein (Ran) GTPase

Spindle morphology changes between meiosis and mitosis driven by CK2 regulation of the Ran pathway

The transition from meiotic divisions in the oocyte to embryonic mitoses is a critical step in animal development. Despite negligible changes to cell size and shape, following fertilization the small, barrel-shaped meiotic spindle is replaced by a large zygotic spindle that nucleates abundant astral microtubules at spindle poles. To probe underlying mechanisms, we applied a drug screening approach using Ciona eggs and found that inhibition of Casein Kinase 2 (CK2) caused a shift from meiotic to mitotic-like spindle morphology with nucleation of robust astral microtubules, an effect reproduced in cytoplasmic extracts prepared from Xenopus eggs. In both species, CK2 activity decreased at fertilization. Phosphoproteomic differences between Xenopus meiotic and mitotic extracts that also accompanied CK2 inhibition pointed to RanGTP-regulated factors as potential targets. Interfering with RanGTP-driven microtubule formation suppressed astral microtubule growth caused by CK2 inhibition. These data support a model in which CK2 activity attenuation at fertilization leads to activation of RanGTP-regulated microtubule effectors that induce mitotic spindle morphology.

LGMD D2 TNPO3-Related: From Clinical Spectrum to Pathogenetic Mechanism

Limb-girdle muscular dystrophies (LGMDs) are clinically and genetically heterogeneous diseases presenting with a wide clinical spectrum. Autosomal dominant LGMDs represent about 10–15% of LGMDs and include disorders due to defects of DNAJB6, transportin-3 (TNPO3), HNRNPDL, Calpain-3 (CAPN3), and Bethlem myopathy. This review article aims to describe the clinical spectrum of LGMD D2 TNPO3-related, a rare disease due to heterozygous mutation in the TNPO3 gene. TNPO3 encodes for transportin-3, which belongs to the importin beta family and transports into the nucleus serine/arginine-rich (SR) proteins, such as splicing factors, and HIV-1 proteins, thus contributing to viral infection. The purpose of this review is to present and compare the clinical features and the genetic and histopathological findings described in LGMD D2, performing a comparative analytical description of all the families and sporadic cases identified. Even if the causative gene and mutations of this disease have been identified, the pathogenic mechanisms are still an open issue; therefore, we will present an overview of the hypotheses that explain the pathology of LGMD D2 TNPO3-related.

CRISPR/Cas9-Induced Mutagenesis Corroborates the Role of Transportin-SR2 in HIV-1 Nuclear Import

To infect nondividing cells, HIV-1 needs to cross the nuclear membrane. The importin transportin-SR2 (TRN-SR2 or transportin-3) has been proposed to mediate HIV-1 nuclear import, but the detailed mechanism remains unresolved. The direct interaction of TRN-SR2 with HIV-1 integrase (IN) has been proposed to drive HIV-1 nuclear import. Alternatively, TRN-SR2 may play an indirect role by mediating nuclear import of cleavage and polyadenylation specificity factor 6 (CPSF6). To unravel the role of TRN-SR2, we designed CRISPR/Cas9 guide RNAs targeting different exons of TNPO3. Although this approach failed to generate full knockouts, monoallelic knockout clones were generated with indel mutations. HIV-1 replication was hampered in those clones at the level of HIV-1 nuclear import without an effect on the cellular distribution of the TRN-SR2 cargoes CPSF6 or alternative splicing factor1/pre-mRNA splicing factor SF2 (ASF/SF2). Recombinant ΔV₁₀₅ TRN-SR2 expressed in clone 15.15 was 2-fold impaired for interaction with HIV-1 IN and classified as an interaction mutant. Our data support a model whereby TRN-SR2 acts as a cofactor of HIV-1 nuclear import without compromising the nuclear import of cellular cargoes. CRISPR/Cas9-induced mutagenesis can be used as a method to generate interface mutants to characterize host factors of human pathogens.

IMPORTANCE Combination antiretroviral therapy (cART) effectively controls HIV-1 by reducing viral loads, but it does not cure the infection. Lifelong treatment with cART is a prerequisite for sustained viral suppression. The rapid emergence of drug-resistant viral strains drives the necessity to discover new therapeutic targets. The nuclear import of HIV-1 is crucial in the HIV-1 replication cycle, but the detailed mechanism remains incompletely understood. This study provides evidence that TRN-SR2 directly mediates HIV-1 nuclear import via the interaction with HIV-1 integrase. The interaction between those proteins is therefore a promising target toward a rational drug design which could lead to new therapeutic strategies due to the bottleneck nature of HIV-1 nuclear import.

HIV-1 Hijacking of Host ATPases and GTPases That Control Protein Trafficking

The human immunodeficiency virus (HIV-1) modifies the host cell environment to ensure efficient and sustained viral replication. Key to these processes is the capacity of the virus to hijack ATPases, GTPases and the associated proteins that control intracellular protein trafficking. The functions of these energy-harnessing enzymes can be seized by HIV-1 to allow the intracellular transport of viral components within the host cell or to change the subcellular distribution of antiviral factors, leading to immune evasion. Here, we summarize how energy-related proteins deviate from their normal functions in host protein trafficking to aid the virus in different phases of its replicative cycle. Recent discoveries regarding the interplay among HIV-1 and host ATPases and GTPases may shed light on potential targets for pharmacological intervention.

Role of Transportin-SR2 in HIV-1 Nuclear Import

The HIV replication cycle depends on the interaction of viral proteins with proteins of the host. Unraveling host-pathogen interactions during the infection is of great importance for understanding the pathogenesis and the development of antiviral therapies. To date HIV uncoating and nuclear import are the most debated steps of the HIV-1 replication cycle. Despite numerous studies during past decades, there is still much controversy with respect to the identity and the role of viral and host factors involved in these processes. In this review, we provide a comprehensive overview on the role of transportin-SR2 as a host cell factor during active nuclear transport.

Changes in Proteome of Fibroblasts Isolated from Psoriatic Skin Lesions

The dermal fibroblasts are in constant contact with the cells of the immune system and skin epidermis. Therefore, they are essential for the development of lesions in psoriasis. The aim of this study was to assess the changes in the proteomic profile of fibroblasts in the dermis of psoriasis patients, and to discuss the most significant changes and their potential consequences. The proteomic results indicate that fibroblast dysfunction arises from the upregulation of proinflammatory factors and antioxidant proteins, as well as those involved in signal transduction and participating in proteolytic processes. Moreover, downregulated proteins in psoriatic fibroblasts are mainly responsible for the transcription/translation processes, glycolysis/ adenosine triphosphate synthesis and structural molecules. These changes can directly affect intercellular signaling and promote the hyperproliferation of epidermal cells. A better understanding of the metabolic effects of the proteomic changes observed could guide the development of new pharmacotherapies for psoriasis.

Inhibitors of the integrase-transportin-SR2 interaction block HIV nuclear import

Background

Combination antiretroviral therapy efficiently suppresses HIV replication in infected patients, transforming HIV/AIDS into a chronic disease. Viral resistance does develop however, especially under suboptimal treatment conditions such as poor adherence. As a consequence, continued exploration of novel targets is paramount to identify novel antivirals that do not suffer from cross-resistance with existing drugs. One new promising class of targets are HIV protein–cofactor interactions. Transportin-SR2 (TRN-SR2) is a β-karyopherin that was recently identified as an HIV-1 cofactor. It has been implicated in nuclear import of the viral pre-integration complex and was confirmed as a direct binding partner of HIV-1 integrase (IN). Nevertheless, consensus on its mechanism of action is yet to be reached.

Results

Here we describe the development and use of an AlphaScreen-based high-throughput screening cascade for small molecule inhibitors of the HIV-1 IN–TRN-SR2 interaction. False positives and nonspecific protein–protein interaction inhibitors were eliminated through different counterscreens. We identified and confirmed 2 active compound series from an initial screen of 25,608 small molecules. These compounds significantly reduced nuclear import of fluorescently labeled HIV particles.

Conclusions

Alphascreen-based high-throughput screening can allow the identification of compounds representing a novel class of HIV inhibitors. These results corroborate the role of the IN–TRN-SR2 interaction in nuclear import. These compounds represent the first in class small molecule inhibitors of HIV-1 nuclear import.

N-terminal half of transportin SR2 interacts with HIV integrase

The karyopherin transportin SR2 (TRN-SR2, TNPO3) is responsible for shuttling specific cargoes such as serine/arginine-rich splicing factors from the cytoplasm to the nucleus. This protein plays a key role in HIV infection by facilitating the nuclear import of the pre-integration complex (PIC) that contains the viral DNA as well as several cellular and HIV proteins, including the integrase. The process of nuclear import is considered to be the bottleneck of the viral replication cycle and therefore represents a promising target for anti-HIV drug design. Previous studies have demonstrated that the direct interaction between TRN-SR2 and HIV integrase predominantly involves the catalytic core domain (CCD) and the C-terminal domain (CTD) of the integrase. We aimed at providing a detailed molecular view of this interaction through a biochemical characterization of the respective protein complex. Size-exclusion chromatography was used to characterize the interaction of TRN-SR2 with a truncated variant of the HIV-1 integrase, including both the CCD and CTD. These experiments indicate that one TRN-SR2 molecule can specifically bind one CCD-CTD dimer. Next, the regions of the solenoid-like TRN-SR2 molecule that are involved in the interaction with integrase were identified using AlphaScreen binding assays, revealing that the integrase interacts with the N-terminal half of TRN-SR2 principally through the HEAT repeats 4, 10, and 11. Combining these results with small-angle X-ray scattering data for the complex of TRN-SR2 with truncated integrase, we propose a molecular model of the complex. We speculate that nuclear import of the PIC may proceed concurrently with the normal nuclear transport.

The HIV-1 integrase mutant R263A/K264A is 2-fold defective for TRN-SR2 binding and viral nuclear import

Transportin-SR2 (Tnpo3, TRN-SR2), a human karyopherin encoded by the TNPO3 gene, has been identified as a cellular cofactor of HIV-1 replication, specifically interacting with HIV-1 integrase (IN). Whether this interaction mediates the nuclear import of HIV remains controversial. We previously characterized the TRN-SR2 binding interface in IN and introduced mutations at these positions to corroborate the biological relevance of the interaction. The pleiotropic nature of IN mutations complicated the interpretation. Indeed, all previously tested IN interaction mutants also affected RT. Here we report on a virus with a pair of IN mutations, IN(R263A/K264A), that significantly reduce interaction with TRN-SR2. The virus retains wild-type reverse transcription activity but displays a block in nuclear import and integration, as measured by quantitative PCR. The defect in integration of this mutant resulted in a smaller increase in the number of two-long terminal repeat circles than for virus specifically blocked at integration by raltegravir or catalytic site mutations (IN(D64N/D116N/E152Q)). Finally, using an eGFP-IN-labeled HIV fluorescence-based import assay, the defect in nuclear import was corroborated. These data altogether underscore the importance of the HIV-IN TRN-SR2 protein-protein interaction for HIV nuclear import and validate the IN/TRN-SR2 interaction interface as a promising target for future antiviral therapy.

Structure of transportin SR2, a karyopherin involved in human disease, in complex with Ran

Transportin SR2 (TRN-SR2) is a β-type karyopherin responsible for the nuclear import of specific cargoes, including serine/arginine-rich splicing factors. The protein has been implicated in a variety of human diseases, including HIV infection, primary biliary cirrhosis and limb-girdle muscular dystrophy 1F. Towards understanding its molecular mechanism, a 2.9 Å resolution crystal structure of human TRN-SR2 complexed with the small GTPase Ran has been determined. TRN-SR2 is composed of 20 α-helical HEAT repeats forming a solenoid-like fold. The first nine repeats form a `cradle' for the binding of RanGTP, revealing similarities but also differences with respect to the related importin 13 complex.

Viral and cellular requirements for the nuclear entry of retroviral preintegration nucleoprotein complexes

Retroviruses integrate their reverse transcribed genomes into host cell chromosomes as an obligate step in virus replication. The nuclear envelope separates the chromosomes from the cell cytoplasm during interphase, and different retroviral groups deal with this physical barrier in different ways. Gammaretroviruses are dependent on the passage of target cells through mitosis, where they are believed to access chromosomes when the nuclear envelope dissolves for cell division. Contrastingly, lentiviruses such as HIV-1 infect non-dividing cells, and are believed to enter the nucleus by passing through the nuclear pore complex. While numerous virally encoded elements have been proposed to be involved in HIV-1 nuclear import, recent evidence has highlighted the importance of HIV-1 capsid. Furthermore, capsid was found to be responsible for the viral requirement of various nuclear transport proteins, including transportin 3 and nucleoporins NUP153 and NUP358, during infection. In this review, we describe our current understanding of retroviral nuclear import, with emphasis on recent developments on the role of the HIV-1 capsid protein.