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The TSC1 tumour suppressor hamartin regulates cell adhesion through ERM proteins and the GTPase Rho

Abstract

Loss of the tumour-suppressor gene TSC1 is responsible for hamartoma development in tuberous sclerosis complex (TSC), which renders several organs susceptible to benign tumours. Hamartin, the protein encoded by TSC1, contains a coiled-coil domain and is expressed in most adult tissues, although its function is unknown. Here we show that hamartin interacts with the ezrin-radixin-moesin (ERM) family of actin-binding proteins. Inhibition of hamartin function in cells containing focal adhesions results in loss of adhesion to the cell substrate, whereas overexpression of hamartin in cells lacking focal adhesions results in activation of the small GTP-binding protein Rho, assembly of actin stress fibres and formation of focal adhesions. Interaction of endogenous hamartin with ERM-family proteins is required for activation of Rho by serum or by lysophosphatidic acid (LPA). Our data indicate that disruption of adhesion to the cell matrix through loss of hamartin may initiate the development of TSC hamartomas and that a Rho-mediated signalling pathway regulating cell adhesion may constitute a rate-limiting step in tumour formation.

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Figure 1: Interaction of hamartin with ERM-family proteins.
Figure 2: Inactivation of hamartin inhibits cell-substrate adhesion.
Figure 3: Hamartin activates the small GTPase Rho.
Figure 4: Activation of Rho by hamartin.
Figure 5: Hamartin domains involved in Rho activation and ERM binding.
Figure 6: Interaction of hamartin with ERM proteins is necessary for activation of Rho.
Figure 7: The ERM-binding domain of hamartin inhibits LPA-induced activation of Rho in Swiss 3T3 fibroblasts.

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Acknowledgements

We thank J. Cheadle for full-length human hamartin cDNA, N. Ito and G. Rubin for Drosophila hamartin cDNA, P. Mangeat for anti-ezrin antibody and pGEX human ERM constructs, J. Gusella and V. Ramesh for the pGEX construct of human NF2 isoform 1, S. Tsukita for mouse anti-chick ERM monoclonal antibody, P. van der Sluijs for pGADGH giantin cDNA and M. Schwartz for pGEX RBD of Rhotekin. R.F.L. thanks H. Paterson for help with preliminary micro-CALI experiments, V. Bragga for the L63 Rho protein, S. Povey, J. Sampson, M. Nellist, J. Connolly and C. Nobes for discussion and technical advice and M. Marsh for critical reading of the manuscript. This work was supported by an MRC Project Grant (to R.F.L.) and grants from NINDS and NCI (to D.G.J.). M.W.J. is supported in part by a USPHS/NIH Molecular and Cellular Neurobiology training grant.

Correspondence and requests for materials should be addressed to R.F.L.

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Lamb, R., Roy, C., Diefenbach, T. et al. The TSC1 tumour suppressor hamartin regulates cell adhesion through ERM proteins and the GTPase Rho . Nat Cell Biol 2, 281–287 (2000). https://doi.org/10.1038/35010550

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