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Nuclear Protein Sorting01:34

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Nuclear protein sorting is the selective trafficking of histones, polymerases, gene regulatory proteins into the nucleus and exporting RNAs and ribosomes to the cytosol. It is a tightly controlled process that regulates gene expression within a cell.
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Proteins targeted to the nucleus carry short stretches of amino acid sequences called the nuclear localization signal or NLS. Classical nuclear localization signals are of two types: monopartite and bipartite NLS. Monopartite classical NLS (cNLS) consists of a single cluster of 4-8 amino acids. Bipartite cNLS consists of two clusters of  2-3 amino acids and a 9-12 residue long proline-rich linker bridging the two clusters. Signal clusters are rich in positively charged amino acids such as...
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Protein Transport into the Inner Mitochondrial Membrane01:34

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Nuclear encoded mitochondrial precursors are imported to the inner membrane in a multistep process involving two separate translocons, TIM22 and TIM23. TIM23 is a cation-selective pore that remains closed by the N terminal segment of the protein. Negative charges on the TIM23 act as a receptor for the incoming precursor, pulling the positively charged matrix-targeting sequence for peptide insertion and translocation.
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Before mRNAs are exported to the cytoplasm, it is crucial to check each mRNA for structural and functional integrity. Eukaryotic cells use several different mechanisms, collectively known as mRNA surveillance, to look for irregularities in mRNAs. Irregular or aberrant mRNA are rapidly degraded by various enzymes. If a defective mRNA escapes the surveillance, it would be translated into a protein which would either be non-functional or not function properly. One of the primary irregularities in...
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Mitochondrial protein import is powered by two distinct energy sources: ATP hydrolysis and electrochemical potential across the inner membrane. Newly synthesized precursors are bound by cytosolic chaperones of the Hsp70 family, which guide them to the import receptors on the mitochondrial surface. Utilizing the energy of ATP hydrolysis, Hsp70 chaperones transfer these precursors to the TOM receptors on the mitochondrial outer membrane.
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Translocation of HIV capsid core through the Nuclear Pore Complex by affinity gradient.

Ivo Melčák1,2, Ryan L Slack1,2, Zachary C Lorson1,2

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The HIV capsid core uses a gradient of binding affinities to phenylalanine-glycine (FG) repeats within the Nuclear Pore Complex (NPC) to ensure unidirectional entry into the host cell nucleus.

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Area of Science:

  • Virology
  • Cell Biology
  • Structural Biology

Background:

  • The HIV capsid core is essential for viral genome delivery into host cells.
  • Nuclear Pore Complex (NPC) translocation is a critical step in HIV replication.
  • The molecular mechanisms governing capsid core transit through the NPC are not fully understood.

Purpose of the Study:

  • To investigate the molecular interactions between the HIV capsid core and phenylalanine-glycine (FG) repeats within the NPC.
  • To elucidate the role of different FG repeat motifs and binding enhancers in capsid translocation.
  • To understand how these interactions facilitate unidirectional nuclear entry.

Main Methods:

  • Biochemical assays
  • Biophysical techniques
  • Structural analysis
  • Quantitative interaction studies

Main Results:

  • HIV capsid (CA) proteins exhibit varying affinities for diverse FG repeats.
  • GLFG motifs of NUP98 show higher affinity to CA than canonical FG/FxFG motifs.
  • A non-canonical FxFG motif in NUP153, termed FG super-motif, demonstrates significantly enhanced binding affinity to CA, further boosted by basic residues.
  • Binding affinity increases with proximity to the NPC's nuclear basket, with a ~1,000-fold difference observed for NUP153's FG super-motif.
  • NPC FG-nucleoporins NUP58 and POM121 act as binding enhancers.

Conclusions:

  • A gradient of avidity, driven by diverse FG motifs and binding enhancers, exists along the cytoplasmic-nuclear axis within the NPC.
  • This avidity gradient potentiates unidirectional HIV capsid translocation into the nucleus.
  • Understanding these interactions offers potential targets for antiviral therapies.