Research Interests
Ubiquitylation is a key regulator of proliferation and differentiation in all
eukaryotes. It is carried out by a cascade of three different classes of
enzymes, E1, E2, and E3. In humans alone, there are 38 E2s and more
than 600 E3s, making ubiquitylation enzymes one of the most abundant and
diverse family of enzymes. Loss of essential ubiquitylation enzymes can
lead to rapid cell cycle arrest.
The importance of ubiquitylation for cell cycle control is underscored by the tight links between deregulated ubiquitylation and tumorigenesis. Losing the activity of the E3 Brca1/Bard1, for example, can cause breast and ovarian cancer. The overexpression of the ubiquitin ligase Mdm2, which targets the tumor suppressor p53 for degradation, is observed multiple types of cancers. Small molecules that would correct the activities of rampant ubiquitylation enzymes would be exciting additions to the currently available set of chemotherapeutics.
Our
ability to exploit the ubiquitin system as a target for drug discovery
has been hampered by our limited knowledge of essential enzymes and
their substrates, and particularly, by our lack of understanding
mechanisms of ubiquitylation.
Current Research Projects
1. Dissecting the ubiquitin code
Ubiquitin
is covalently linked to lysine residues in substrate proteins. The
transfer of a single ubiquitin moiety (monoubiquitylation) usually
results in changes of proteins interactions. The modification of the
substrate-linked ubiquitin with further ubiquitin molecules leads to the
generation of polymeric ubiquitin chains. These chains can be linked
through the N-terminus or through each of the seven lysine residues of
ubiquitin, resulting in chains of different structure and function.
K48-linked chains, for example, trigger degradation by the 26S
proteasome, whereas K63-linked chains recruit binding partners into
multimeric protein assemblies. For many other chain topologies, however,
very little is known about substrates, enzymes, and functions.
We
have discovered the K11-linked ubiquitin chain as an essential
regulator of cell division in higher eukaryotes. K11-linked ubiquitin
chains are assembled during mitosis by the ubiquitin ligase APC/C, a
finding we recently confirmed in cells using linkage-specific
antibodies. We isolated the responsible E2 enzymes for K11-linkage
formation, Ube2C/UbcH10 for chain initiation and Ube2S for chain
elongation. We are currently investigating the mechanisms of specific
and regulated K11-linked chain formation by these enzymes.
2. Ubiquitin-dependent regulation of proliferation and differentiation
Substrate specificity of ubiquitylation depends on ~600-1000 so-called
E3 enzymes, and for the majority of these enzymes, functions or
substrates remain unknown. Ubiquitylation often is reversible; the
modification then has to be removed by one out of ~100 deubiquitylating
enzymes (DUBs), most of which remain uncharacterized. We have developed
a ubiquitin-related siRNA library and a robust siRNA-screening platform
to isolate new E3s and DUBs with important roles in proliferation and
differentiation. Using this setup, we could identify an important role
for ubiquitin in regulating the composition and function of the
spliceosome. Loss of this pathway led to incorrect tubulin splicing,
inaccurate spindle formation and reduced sensitivity to treatment with
the chemotherapeutic taxol.
3. Small molecule discovery to modulate the activity of ubiquitylation enzymes
We
are combining our biochemical insight and the siRNA-based enzyme
discovery to identify targets for the development of small molecules
against ubiquitylation enzymes. We are screening for small molecules
that could either active or inhibit the activity of these enzymes,
providing a proof-of-principle that ubiquitylation enzymes are
attractive drug targets.