We are analyzing the control of the cell cycle at genetic, cellular and biochemical levels and are interested in the integration of the cell cycle with developmental processes. We are using the whole spectrum of the model organism Drosophila and cell culture systems.
In our studies, we have uncovered important mechanisms that regulate entry into and exit from mitosis. We will continue working on the fascinating mitotic process. In addition, we are interested in the mechanisms that regulate the G1-S transition and would like to understand how special cell cycles are controlled. Special cell cycle types, for example endoreduplication cycles are found in many multicellular organisms, including humans.
Our results will help to increase our knowledge of normal cell proliferation control mechanisms and thereby will help to understand what goes wrong in uncontrolled cell proliferation diseases like cancer.



Ongoing projects and interests

Regulation of the Anaphase-Promoting-Complex/Cyclosome (APC/C) and its inhibitor Rca1

Protein destruction mediated by the Rca1-SCF-complex

Regulation of endoreduplication cycles

The fastest eukaryotic cell cycle on earth – How is it controlled?

Methods being used in the lab

Regulation of the Anaphase-Promoting-Complex/Cyclosome (APC/C) and its inhibitor Rca1

The APC/C is a multisubunit ubiquitin-Ligase that is used to mark proteins for destruction during a cell cycle. The irreversible destruction of proteins is a common theme in cell cycle regulation. It ensures that the cell cycle goes in one direction and helps to reinforce transition steps. The attachment of ubiquitin to target proteins marks them for deletion in the proteasome. For an orderly cell cycle, the destruction of important cell cycle regulators has to be tightly controlled during the cycle.
The APC/C ubiquitin ligase is normally active during mitosis and G1 where it is responsible for the degradation of many cell cycle proteins, like cyclins, securing and many others. However, cyclins are required for entry into mitosis and APC/C activity has to be restrained during S and G2 phase to allow cyclin accumulation.


We have identified the gene rca1 as an essential inhibitor of the APC/C. In the absence of Rca1 function, cells fail to divide because cyclins, that are required for entry into mitosis are untimely degraded before mitosis by the APC/C. Thus, Rca1 is required to inhibit the APC/C before mitosis. Rca1 shares limited sequence homology with the APC/C inhibitors of the vertebrate Emi-family. For these proteins, it was proposed that they work by inhibiting the APC/C as a pseudosubstrate inhibitor. We would like to establish if Rca1 works in a similar manner and how APC/C inhibition is achieved. In addition, Rca1 itself has to be regulated during the cell cycle to relieve APC/C inhibition when the ubiquitin ligase should be active. We could observe that Rca1 is itself degraded during G1 and now would like to understand how the degradation is achieved and regulated.


Besides its role during the cell cycle, the APC/C has been reported to be active and required in cells that have exited the cell cycle. By using APC/C sensors and APC/C mutations or RNAi knockdown procedures we would like to see if other, non cell-cycle functions of the APC/C are required.


Protein destruction mediated by the Rca1-SCF-complex

Another class of ubiquitin ligases that play important roles during the cell cycle is the SCF-complex which is composed of several proteins, including Skp1, Cul1, an F-box-containing protein and a protein that has ubiquitin-ligase activity. SCF complexes are evolutionarily conserved among eukaryotes; they are found in organisms from yeast to humans. The F-box proteins contain at least two domains: an F-box responsible for binding to Skp1 and another protein-protein interaction domain which is believed to interact with the substrate protein.

The above-mentioned Rca1 protein contains the F-box motif and we could show that Rca1 interacts with Skp and cullin proteins. The F-box motif is not required for APC/C inhibition during the G2-phase, but Rca1 lacking the F-box is unable to fully complement rca1 function. Cells that only have Rca lacking the F-box fail to proliefate normally. We are interested what the function such Rca1 containing SCF-complexes have and what proteins might be targeted for degradation by the Rca1-SCF-complex. Both genetic and biochemical approaches are currently being used to identify these novel function of Rca1


Regulation of “special” cell cycle types

The most common cell cycle type consists of 4 Phases: Mitosis when cell division takes place is usually followed by a so called “gap”-Phase, G1 before DNA-replication is initiated during S-phase that is followed by another gap-Phase, G2.
In multicellular organisms many “special” cell cycle types can be observed as well, especially during development. In Drosophila, the first cycles consist solely of S and M-Phases and slightly later a G1-phase is still not present in the division pattern. Endoreduplication cycles (see below) and meiosis, when germ cells are generated are fascinating deviations from the normal cell cycle pattern that we would like to understand.


Regulation of endoreduplication cycles

The precise duplication of the genome is crucial for the survival of any organism. In multicellular organisms genome instability potentially gives rise to cancer and thus compromises the life of the whole organism. To maintain the integrity of the genome, DNA replication and mitosis must be coordinated during cell division cycles so that DNA replication occurs only once per cycle and mitosis only after complete duplication of the genome. To avoid re-replication events, a network of proteins ensures that cells acquire the license for DNA replication only in a specific phase of the cell cycle.


While re-replication without an intervening mitosis is strictly prevented during cell division cycles, this process is enforced during endoreplication cycles. This cell cycle variant is prominently found during plant and invertebrate development, but some mammalian cell types, like megakaryocytes or trophoblast giant cells, are also known to undergo endoreplication cycles. Endoreplicating cells undergo repeated rounds of DNA synthesis that are separated by distinct Gap phases, but never enter mitosis.


Despite the absence of mitosis, the Anaphase-Promoting Complex is required for endoreduplication cycles. However, the degradation of mitotic substrates like cyclins is of no importance here. Instead, APC/C is required for the degradation of an inhibitor, geminin that regulates the assembly of DNA-replication origns. When the APC/C is inhibited, Geminin accumulates and prevents reassembly of DNA-replication origins.


Repeated rounds of S- and Gap phases are regulated by distinct pathways thar result in oscialltions of cell cycle regulators. At the heart of the system is Cyclin E, an unstable protein that requires transcriptional input from the transcription factor E2F. Once E2F can accumulate, it triggers DNA-replication and this in turn results in E2F degradation, hence reduced CycE. The APC/C is inhibited by CycE and only after a drop in CycE levels, APC/C is able to degrade geminin relieving the inhibition in DNA-Origins assembly


The fastest eukaryotic cell cycle on earth – How is it controlled?

The first divisions after fertilization are nuclear divisions that occur in a common cytoplasm. These cycles occur in an almost synchronous manner, consist of S- and M-Phase only are the fastest cell cycle known for eucaryotes.


The speed and synchrony as well as the relative big size of the embryo make this an ideal model to follow cell cycle events in real time. This is possible through the use of fluorescently marked proteins (see this years Nobel Prize in Chemisty) that are attached to proteins that bind to DNA (in Cyan) or Tubulin (red). In the movie on the right, three cycles from a part of an early Drosophila embryo are shown.

The earliest cycles during embryogenesis are not easily explained by standard cell cycle models. Usually, a cell cycle is characterized by oscillations in cell cycle components or their activity. For example, Cdk1, a kinase that triggers mitosis, is only active shortly before mitosis when it promotes mitosis and needs to be inactivated when cells exit mitosis. However, in the early nuclear cycles, no fluctuations in Cdk1 activity can be observed. It remains to be investigated how these cycles are coordinated.