Richard Treisman

 

RPEL proteins, actin dynamics, and cytoskeletal regulation

Our laboratory is characterising a novel signalling pathway, the Rho-actin pathway, which allows the coordination of protein activity with the dynamics of the actin cytoskeleton. The targets of the Rho-actin pathway in turn play both direct and indirect roles in the control of cytoskeletal behaviour and biological processes dependent on it such as cell morphology and adhesion, and cell migration and metastasis.

Rho-actin target proteins all contain a novel G-actin binding element, the RPEL motif, and our current model is that G-actin regulates their activity by competing for effector proteins that they require for function. There are four RPEL families.

(i) The two MRTFs are transcriptional coactivators for the SRF transcription factor whose nuclear accumulation is controlled by G-actin [1,2]. They control hundreds of genes, many of which build or regulate the actin cytoskeleton, and as a result, MRTF-null cells exhibit adhesion, motiliy and metastasis defects [3]. In addition, within the nucleus G-actin controls their ability to activate transcription by a means that remains to be elucidated [4]. We are using genomic and biochemical approaches to study these events.

(ii) The four Phactr proteins are regulatory subunits of the PP1 phosphatase. G-actin controls both their subcellular localisation - at nucleus, cytoplasm or cell membrane - and their interaction with the PP1 catalytic subunit. Differentially localised in the cell through an unknown mechanism, they control motility, invasiveness, and cytoskeletal dynamics [5,6]. We are beginning to identify potential substrates, several of which appear to be localised at the cell surface with potential roles in cell adhesion and cytoskeletal dynamics. Future work will build on this to develop a complete picture of their downstream signalling; its biological significance, and the significance of the G-actin interaction.

(iii) Two families of Rac/Cdc42 GAP proteins, both of which contain multiple protein interaction domains in addition to the RPEL and GAP domains. Here G-actin binding antagonises binding of the GTPases to the GAP domain, and also likely to control their association with cell-cell junctions [J. Diring and RT, in preparation]. An initial analysis of the interacting proteins for one of these families has identified components of the F-actin nucleation machinery.

We aim to study both the processes controlled by RPEL proteins and the context in which they operate, and number of different new research directions are being developed. Examples of potential PhD projects include studies of RPEL-interacting proteins and cytoskeletal regulation, RPEL protein subcellular localisation, and the control of these events by cellular signals, with a focus on the functional role of G-actin interactions. We also wish to develop methods to visualise where in the cell G-actin/RPEL proteins interactions occur, how these interactions change in response to external signalling, and whether the RPEL/G-actin interaction can be used to monitor G-actin concentration in vivo.

Projects utilise a wide variety of experimental approaches including molecular cell biology, functional genomics and global analysis of gene expression, siRNA screening, biochemical and structural studies, and mouse models. The precise project pursued for the PhD will be decided on in consultation with the supervisor.

1. Vartiainen, M. K., Guettler, S., Larijani, B. and Treisman, R. (2007)
Nuclear actin regulates dynamic subcellular localization and activity of the SRF cofactor MAL.
Science 316: 1749-1752. PubMed abstract

2. Pawłowski, R., Rajakylä, E. K., Vartiainen, M. K. and Treisman, R. (2010)
An actin-regulated importin α/β-dependent extended bipartite NLS directs nuclear import of MRTF-A.
EMBO Journal 29: 3448-3458. PubMed abstract

3. Medjkane, S., Perez-Sanchez, C., Gaggioli, C., Sahai, E. and Treisman, R. (2009)
Myocardin-related transcription factors and SRF are required for cytoskeletal dynamics and experimental metastasis.
Nature Cell Biology 11: 257-268. PubMed abstract

4. Gualdrini, F., Esnault, C., Horswell, S., Stewart, A., Matthews, N. and Treisman, R. (2016)
SRF co-factors control the balance between cell proliferation and contractility.
Molecular Cell 64: 1048-1061. PubMed abstract

5. Wiezlak, M., Diring, J., Abella, J., Mouilleron, S., Way, M., McDonald, N. Q. and Treisman, R. (2012)
G-actin regulates the shuttling and PP1 binding of the RPEL protein Phactr1 to control actomyosin assembly.
Journal of Cell Science 125: 5860-5872. PubMed abstract

6. Mouilleron, S., Wiezlak, M., O'Reilly, N., Treisman, R. and McDonald, N. Q. (2012)
Structures of the Phactr1 RPEL domain and RPEL motif complexes with G-actin reveal the molecular basis for actin binding cooperativity.
Structure 20: 1960-1970. PubMed abstract