Mikrobiologie (Römisch)

The research field in short

Secretory and transmembrane proteins constitute a third of the proteome. In eukaryotes secretory protein biogenesis starts with transport across or integration into the endoplasmic reticulum (ER) membrane. The majority of secretory proteins enter the ER through the Sec61 channel. The channel can also function during retrograde transport of misfolded proteins from the ER to the cytosol for degradation by proteasomes (ER-associated degradation). We use the budding yeast Saccharomyces cerevisiae as model organism to study these processes with a combination of yeast genetics, protein biochemical techniques, and cell-free assays.

Detailed description of research questions

Proteins are targeted to the ER by an amino-terminal hydrophobic signal sequence which also triggers opening of the Sec61 channel. Most signal sequences are good at targeting, but suboptimal for translocation. This generates a window for regulation of ER protein import. Protein transport through the Sec61 channel, however, has been studied for decades using only a handful of reporter proteins with signal sequences that are optimal for translocation. We therefore know little about regulated secretory protein biogenesis.
 
The Sec61 channel consists of 3 proteins named Sec61alpha, Sec61beta, Sec61gamma in mammals (Sec61, Sbh1, Sss1 in yeast). Sec61beta/Sbh1 is the only non-essential channel subunit. We have shown that virulence factor biogenesis in the WHO top priority fungal pathogen Cryptococcus neoformans is controlled by Sbh1. It is also involved in pathogenesis of the world's economically most important crop pathogen, the rice blast fungus Magnaporthe oryzae. 
 
Sbh1/Sec61beta is peripherally associated with the Sec61 channel and consists of an intrinsically unstructured N-terminal cytosolic domain which is poorly conserved and contains multiple phosphorylation sites, followed by 15 conserved amino acids and a single conserved transmembrane helix. We are aiming to understand how Sbh1/Sec61beta contributes to recognition of secretory proteins and their transport through the Sec61 channel.
 
The Sec61 channel cooperates with accessory protein complexes in the ER membrane during ER protein import. One such accessory, the mammalian translocon-associated protein (TRAP) complex, was thought to be metazoan-specific. We found that one of the Sbh1-dependent ER-import substrates, Irc22, is a homolog of the TRAPalpha subunit. We are now investigating its role in ER protein import in yeast.
 
In collaboration with structural biology and bioinformatics labs we are also addressing the effects of Sbh1 and Irc22 on Sec61 channel structure.
 
We have shown that cytosolic proteasomes can bind to the Sec61 channel via their 19S regulatory particles and that this interaction is critical for extraction of a misfolded soluble secretory protein from the ER and its degradation. Other pathways for misfolded proteins including the Hrd1 ubiquitin ligase have been described in the literature. We are currently investigating the contribution of the Sec61 channel to export and ER-associated degradation of misfolded membrane proteins.

Key Collaborators:
Prof. Volkhard Helms, Bioinformatics, Saarland University
Prof. Maya Schuldiner, Dept. of Genetics, Weizmann Institute, Israel