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ER membrane protein complex required for nuclear fusion

D T Ng1, P Walter

  • 1Department of Biochemistry and Biophysics, University of California Medical School, San Francisco, 94143-0448, USA.

The Journal of Cell Biology
|February 1, 1996
PubMed
Summary

Researchers identified key proteins, Sec63p, Sec71p, and Sec72p, essential for nuclear fusion (karyogamy) in yeast. These proteins, working with Kar2p, mediate the fusion of nuclear membranes during cell mating.

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Area of Science:

  • Cell Biology
  • Molecular Biology
  • Genetics

Background:

  • Yeast cells (Saccharomyces cerevisiae) fuse during mating to form diploid cells.
  • Karyogamy, the fusion of two haploid nuclei, is a critical step in yeast mating.
  • The ER luminal protein Kar2p (BiP) is known to be involved in nuclear membrane fusion, but its mechanism of action was unclear due to its luminal localization.

Purpose of the Study:

  • To elucidate the mechanism of nuclear membrane fusion during yeast karyogamy.
  • To identify the specific roles of ER membrane proteins, particularly Sec63p, in the karyogamy process.
  • To investigate the interaction between Kar2p and Sec63p in nuclear fusion.

Main Methods:

  • Isolation and characterization of novel sec63 mutant alleles.
  • Genetic analysis of genes encoding Sec63p-associated proteins (Sec71p, Sec72p).
  • Analysis of mutant alleles in other protein translocation genes (sec61, sec62) and a suppressor mutant (sos1-1).

Main Results:

  • Novel sec63 mutant alleles exhibited severe karyogamy defects.
  • Disruption of Sec71p and Sec72p genes also led to karyogamy defects.
  • Mutants affecting protein translocation (sec61, sec62) did not show karyogamy defects, suggesting a direct role for Sec63p complex.
  • A suppressor mutant (sos1-1) corrected translocation defects but not karyogamy defects.

Conclusions:

  • Sec63p, Sec71p, and Sec72p play a direct and essential role in nuclear membrane fusion during yeast karyogamy.
  • These proteins likely form a complex mediating nuclear fusion, requiring the ER luminally associated Kar2p.
  • The findings clarify the mechanism of nuclear fusion, highlighting a direct function of ER/nuclear membrane proteins.

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