|M.Sc Student||Maya Tal|
|Subject||Interactions of the CSN (COP9 Signalosome) with Components|
of the Ubiquitin System in Yeast
|Department||Department of Biology||Supervisor||Full Professor Glickman Michael|
The COP9 signalosome (CSN complex), the lid subcomplex of the proteasome and translational initiation factor 3 (eIF3) are 3 genetically and structurally homologous complexes which mediate different regulatory pathways. The lid and the CSN complex are highly homologous.
The CSN complex which was originally described as an essential regulator of photomorphogenesis in Arabidopsis was later found in animal systems which suggested that the complex has significance beyond light signaling. The molecular weight of the CSN complex was found to be about 500kD, containing 8 different proteins.
Candidates for CSN subunits were also identified in Saccharomyces cerevisiae (budding yeast), a single cell eukaryote. Csn9, Csn10, Pci8/Csn11, Csn12, Csn5/Jab1 and Csi1 were found to form a complex clustered arround Csn12, in a similar manner to the CSN from other sources.
CSN is able to remove Rub1 from Cdc53 (the major component of a E3 ubiquitin ligase enzyme). This deRubylation activity was mapped specifically to the Csn5 subunit, though it was shown that Csn5 does not act as a deRubylase by it self but only within the context of the CSN complex.
In this work we describe how the CSN complex in S.cerevisiae physically interacts with the proteasome complex in that organism. The interaction characterized in this work is shown to be similar to the interaction between the proteasome and its lid subcomplex. In addition, we also show that this physical interaction induces CSN-like properties on the proteasome by conferring onto the protreasome the ability to carry out deRubylation. We mapped this new enzymatic activity to Csn5 incorporated into intact proteasome. Furthermore, we show that the removal of Csn5 from the proteasome does not harm other known proteasome properties such as deubiquitination or peptidase activities.
While trying to understand the role of CSN in regulating the ubiquitin system we found a link between the CSN and subunits of the Cullin based E3 ubiquitination complex. We show that this interaction has an affect on the pheromone response pathway. This discovery opens up new possibilities for understanding the essence of the CSN complex.
It has been known that CSN affects many processes within the cell, but the exact position of this complex within those pathways was unknown. Our study could bring new insights as to the mode of CSN operation in pheromone response pathway, as well as other cellular pathways.