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Unconventional protein secretion triggered by nutrient starvation.

David Cruz-Garcia1, Vivek Malhotra2, Amy J Curwin1

  • 1Centre for Genomic Regulation, The Barcelona Institute of Science and Technology, 08003 Barcelona, Spain; Universitat Pompeu Fabra (UPF), 08002 Barcelona, Spain.

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Summary

Eukaryotic cells can secrete proteins lacking signal sequences via a novel pathway. This unconventional protein secretion (UPS) pathway utilizes a compartment for unconventional protein secretion (CUPS) for sorting and release.

Keywords:
Acb1ESCRTsGRASPIL-1βInsulin degrading enzymeSOD1

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Area of Science:

  • Cell Biology
  • Molecular Biology
  • Biochemistry

Background:

  • Eukaryotic cells conventionally secrete proteins via the endoplasmic reticulum (ER) and Golgi apparatus, requiring a signal sequence.
  • Many proteins, including superoxide dismutase (SOD1) and acyl-CoA binding protein (Acb1), are secreted despite lacking signal sequences, challenging conventional models.
  • The mechanisms governing the secretion of signal sequence-deficient proteins remain largely unknown.

Purpose of the Study:

  • To elucidate the common secretion pathway for signal sequence-lacking proteins like SOD1 and Acb1.
  • To identify the molecular determinants and cellular compartments involved in this unconventional protein secretion (UPS) pathway.
  • To understand the functional implications of UPS in cellular processes, particularly under nutrient starvation.

Main Methods:

  • Investigated the secretion of SOD1 and Acb1 under nutrient starvation conditions.
  • Identified a di-acidic motif crucial for the capture of these proteins into a specific cellular compartment.
  • Characterized the compartment for unconventional protein secretion (CUPS) as a key sorting station, derived from Golgi and endosomal membranes.

Main Results:

  • SOD1 and Acb1 are secreted via a type III unconventional pathway upon nutrient starvation.
  • A di-acidic motif facilitates the capture of these proteins into the CUPS.
  • CUPS acts as a sorting station for signal sequence-lacking proteins before their extracellular release.
  • This pathway bypasses the Sec61 translocon for ER entry and COPII/COPI coats for intracellular transport.

Conclusions:

  • A novel, signal sequence-independent secretion pathway exists in eukaryotic cells.
  • This unconventional protein secretion (UPS) pathway relies on the CUPS compartment for sorting and transport.
  • UPS provides an alternative route for protein secretion, particularly important under specific cellular conditions like nutrient starvation.