Initial characterization of the nascent polypeptide-associated complex in yeast

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• The nascent polypeptide-associated complex (NAC) is involved in protein targeting.
• In yeast Saccharomyces cerevisiae, NAC consists of alpha, beta1, and beta3 subunits encoded by EGD2, EGD1, and BTT1 genes.
• Previous in vitro studies proposed NAC prevents mistargeting of non-secretory polypeptides.

Purpose of the Study:

• To investigate the in vivo function and subunit interactions of yeast NAC.
• To determine the consequences of NAC subunit deletions on cellular viability and protein targeting.
• To explore potential compensatory mechanisms in yeast lacking NAC.

Main Methods:

• Analysis of NAC subunit composition and ribosome binding in wild-type and mutant yeast strains.
• Assessment of cell growth and viability at different temperatures and media conditions.
• In vivo assays to monitor the targeting of signal-less invertase.

Main Results:

• Yeast NAC binds to ribosomes via beta-subunits in a salt-sensitive manner, near nascent polypeptides.
• NAC subunit composition is variable, and deletions lead to downregulation of remaining subunits.
• Cells lacking both beta-subunits are inviable at 37°C, suggesting alpha-subunit toxicity.
• Absence of NAC does not lead to mistargeting of signal-less invertase in vivo.
• Yeast strains lacking one, two, or all three NAC subunits show no drastic phenotype at optimal growth conditions.

Conclusions:

• Yeast NAC exhibits flexible subunit composition and essential ribosome association.
• The lack of severe phenotypes in NAC-deficient yeast suggests robust cellular tolerance or compensatory pathways.
• The proposed in vitro function of NAC in preventing non-secretory polypeptide mistargeting may not be critical in vivo for yeast.