Sorting of soluble ER proteins in yeast

Area of Science:

• Cell biology
• Molecular genetics
• Protein trafficking

Background:

• Secretory proteins are sorted in animal cells via the C-terminal KDEL sequence for retention in the endoplasmic reticulum (ER).
• The ER functions as a quality control and protein folding compartment, necessitating mechanisms to prevent aberrant secretion.

Purpose of the Study:

• To investigate if yeast possesses a similar protein sorting system for ER retention.
• To identify the specific C-terminal signal recognized by the yeast ER retrieval system.
• To screen for mutants defective in this protein sorting pathway.

Main Methods:

• Construction of invertase fusion proteins containing the HDEL signal.
• Analysis of protein secretion and modification in wild-type and mutant yeast strains.
• Genetic screening to identify mutants with defects in ER protein retention.

Main Results:

• Yeast utilizes an HDEL C-terminal signal, analogous to the KDEL signal in animal cells, to retain proteins.
• Invertase fusion proteins with the HDEL signal are retained intracellularly and undergo partial Golgi modification.
• A genetic screen identified over 60 mutants, with eight being alleles of the 'erd1' gene, which are defective in ER protein retention and secrete the fusion protein.
• The 'erd1' mutants exhibit normal growth at 30°C but fail to retain the HDEL-tagged fusion protein.

Conclusions:

• Yeast possesses a conserved protein sorting mechanism for ER retention, employing the HDEL signal.
• ER retrieval from the Golgi is a key mechanism for maintaining protein homeostasis within the ER.
• The 'erd1' gene is crucial for the yeast HDEL retrieval pathway, and its disruption leads to protein secretion.