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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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Nuclear protein sorting regulates nucleus composition and gene expression, crucial for determining the fate of a eukaryotic cell. Hence, the entry and exit of molecules across the nuclear envelope is a tightly controlled process. Nuclear protein sorting can be inhibited by one of the following ways: 1) masking cargo signal sequences, 2) modifying the nuclear receptor's affinity for cargo, 3) controlling the nuclear pore size, 4) retaining the cargo during its transit to the cytosol or the...
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Mitochondria are double-membrane organelles of the eukaryotes involved in cellular metabolism, signaling, ATP synthesis, and programmed cell death.  Each of these processes requires specific proteins and enzymes that must be correctly sorted to the right mitochondrial subcompartment for the proper functioning of the organelle.
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Mitochondrial precursors are partially unfolded or loosely folded polypeptide chains. Newly synthesized precursors are inhibited from spontaneously folding into their native conformation by the cytosolic chaperones, heat shock proteins 70 (Hsp70), and mitochondrial import stimulation factors (MSFs). Precursors bound to MSFs are guided to the TOM70-TOM37 receptors, while precursors bound to Hsp70  chaperones are targetted to TOM20-TOM22 receptor complexes.
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PNPASE regulates RNA import into mitochondria.

Geng Wang1, Hsiao-Wen Chen, Yavuz Oktay

  • 1Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles, CA 90095, USA.

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Polynucleotide phosphorylase (PNPASE) unexpectedly mediates nuclear-encoded RNA import into mitochondria. This discovery reveals PNPASE

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Area of Science:

  • Mitochondrial biology
  • Molecular genetics
  • RNA processing

Background:

  • RNA import into mammalian mitochondria is crucial for mitochondrial genome expression.
  • The mechanisms and factors governing mitochondrial RNA import remain largely unknown.
  • Polynucleotide phosphorylase (PNPASE) was previously localized to the mitochondrial intermembrane space.

Purpose of the Study:

  • To investigate the role of PNPASE in the import of nuclear-encoded RNAs into the mitochondrial matrix.
  • To elucidate the function of PNPASE in mitochondrial RNA regulation.

Main Methods:

  • Localization studies of PNPASE within mitochondria.
  • Analysis of mitochondrial RNA processing and import upon PNPASE reduction.
  • Identification of interactions between PNPASE and imported RNAs.
  • Isolation of a mitochondrial RNA targeting signal.

Main Results:

  • PNPASE reduction led to impaired mitochondrial RNA processing and accumulation of polycistronic transcripts.
  • The import of specific RNAs (RNase P, 5S rRNA, MRP RNAs) was dependent on PNPASE expression.
  • PNPASE demonstrated separable RNA processing and import activities.
  • A novel mitochondrial RNA targeting signal was identified, facilitating PNPASE-dependent RNA import.

Conclusions:

  • PNPASE plays a critical, previously unrecognized role in mediating the translocation of nuclear-encoded RNAs into the mitochondrial matrix.
  • PNPASE functions in both RNA processing and import, suggesting a dual role in mitochondrial RNA regulation.
  • The findings uncover a new pathway for RNA import into mitochondria, highlighting PNPASE as a key regulator.