Home Faculty and Research Faculty by Name Michael Rape

Michael Rape

Assistant Professor of Cell and Developmental Biology*
*And Affiliate, Division of Biochemistry and Molecular Biology.

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Research Interests

During the development of multicellular organisms, proliferation of stem cells and their withdrawal from the cell cycle during differentiation are tightly coordinated. Misregulated cell division or differentiation can have severe consequences, such as cancer or organ failure, respectively. A key regulatory mechanism, that governs cell cycle progression and is often compromised in disease, is the modification of proteins with ubiquitin and their subsequent degradation by the 26S proteasome. We are studying the mechanisms of protein ubiquitination and degradation, and how ubiquitination is employed to control cell division and differentiation.

Current Projects

1. Mechanisms of ubiquitination and degradation

Proteins are tagged for degradation by the 26S proteasome by attachment of a ubiquitin chain that contains at least four ubiquitin molecules. The formation of such a ubiquitin chain can occur with different degrees of processivity. A "processive" substrate can obtain a ubiquitin chain in a single binding event to its ubiquitin ligase (which catalyzes ubiquitinations). By contrast, a "distributive" substrate frequently dissociates during ubiquitin chain formation. Such distributive substrates undergo multiple rounds of association and dissociation before a ubiquitin chain of sufficient length has been formed.

Once dissociated, partially ubiquitinated distributive substrates are prone to deubiquitination by specific enzymes. The human genome contains about 80 different deubiquitinating enzymes (DUBs). DUBs add an important layer of regulation to the timing and specificity of ubiquitin-mediated proteolysis.

We have previously shown that the anaphase-promoting complex (APC), a ubiquitin ligase that is essential for human cell cycle progression, ubiquitinates substrates with different degrees of processivity to establish a sequence of degradation events during mitosis. We are currently interested in the mechanism of how the processivity of ubiquitination is established by the APC. Furthermore, we would like to identify deubiquitinating enzymes that are specific for distributive APC-substrates and elucidate their effect on cell cycle control. Finally, we would like to understand how certain substrates can circumvent the sequential degradation based on processivity, and why this is important for the cell cycle

2. Cell cycle control by the anaphase-promoting complex

Degradation of proteins by the APC is essential during cell division and differentiation. Substrates that are degraded by the APC during mitosis include the key cell cycle regulators cyclin A, cyclin B, and securin. A failure of APC-activity arrests cells in mitosis, whereas an increased level of APC-activity has been associated with the generation of aneuploidy and cancer. It is therefore important to decipher mechanisms that regulate APC during the cell cycle and during differentiation and keep its activity in an optimal range. We use both biochemical and cell biological approaches to identify and characterize those regulatory processes.

It is furthermore highly desirable to identify substrates that are degraded by the APC. Those substrates are likely to be important for controlling the cell cycle or differentiation, and they will point us to processes that are regulated by APC. It is likely that many APC-substrates remain to be discovered. We have set up an experimental system that allows us to discover novel human APC-substrates at different stages of the human cell cycle. We are characterizing both the cellular function of those substrates and how their degradation affects cell cycle progression.

3. Control of cell differentiation by protein ubiquitination and degradation

Initiation of cellular differentiation and exit from the cell cycle has to be tightly controlled. Premature cell cycle exit can cause reduced organ size or organ failure. Protein degradation has been shown to be a key regulator to establish the irreversible nature of the transition from proliferation to differentiation. We are interested in ubiquitin ligases that are involved in the initiation of differentiation and in their respective regulators. To this end, we establish biochemically tractable extract systems, that allow the purification and detailed analysis of ubiquitin ligases during differentiation. Furthermore, we use RNAi-based screening combined with in vivo-sensors of ubiquitin ligases to identify novel regulators of these processes. Our experiments will initially focus on the anaphase-promoting complex, which has been shown to be important for neuronal differentiation, but will subsequently be extended to additional ubiquitin ligases.

Selected Publications

Rape, M., Reddy, S.K. and Kirschner, M.W. (2006). The processivity of multiubiquitination by the APC determines the order of substrate degradation. Cell 124, 89-103.

Richly, H.*, Rape, M.*, Braun, S., Rumpf, S., Hoege, C., and Jentsch S. (2005). A series of ubiquitin binding factors connects CDC48/p97 to substrate multiubiquitylation and proteasomal targeting. Cell 120, 73-84.

Rape, M. and Kirschner, M.W. (2004). Autonomous regulation of the anaphase-promoting complex couples mitosis to S-phase entry. Nature 432, 588-595.

Rape, M., Hoppe, T., Gorr, I., Kalocay, M., Richly, H., and Jentsch S. (2001). Mobilization of processed, membrane-tethered SPT23 transcription factor by CDC48. Cell 107, 667-677.

* These authors contributed equally to this work.

Last Updated 2006-10-05