|Ph.D Thesis||Department of Biology|
|Supervisors:||Prof. Glickman Michael|
|Distinguished Prof. Ciechanover Aaron|
|Full Thesis text|
The ubiquitin proteasome pathway is responsible for removal of the vast majority of intracellular proteins. At the core of the pathway is the 26S proteasome - a huge multisubunit proteolytic complex. In order to be degraded, a protein substrate is marked with polyubiquitin, and delivered to the proteasome. The delivery mechanism of polyubiquitin-tagged substrates to the proteasome is not fully understood. There is evidence that shuttle proteins pick up tagged substrates in the cell and deliver them to the proteasome. Listed among the proposed shuttles is Rpn10, which also functions as an integral proteasome subunit. Structurally unrelated shuttles termed Ubl-UBA proteins function alongside Rpn10 with some degree of redundancy.
We evaluated the genetic and biochemical interactions between Rpn10 and Ubl-UBA delivery proteins, and uncovered a role of proteasome-unincorporated (free) Rpn10 in sequestering the ubiquitin-like (Ubl) domain of one such shuttle, Dsk2, from the proteasome. Dsk2 overdose causes defects in the ubiquitin-proteasome pathway such as stabilization of select substrates and accumulation of K48-tagged polyubiquitinated substrates in the cytoplasm, eventually leading to death. Hence the need to limit its availability. By analyzing the profile of polyUb conjugates directly from cell extract we found that Dsk2 is able to alter the ubiquitidome.
The ubiquitidome emerges as a dynamic entity. We propose that Dsk2 and other proteins that have the capacity to stabilize, trim or otherwise to remodel ubiquitin chains will serve as additional layers of regulation. By raising the threshold for binding of some of these polyUb-binding proteins, such as Dsk2, to the proteasome we propose that Rpn10 may assist in sensing discriminating among the multitude of potential degradation targets.