Ubiquitin in signal transduction and cell transformation

Since the discovery of ubiquitin-dependent protein degradation almost two decades ago, great strides have been made towards a detailed understanding of the biochemistry of this process (reviewed in [1-3]). It was, however, only in recent years that the physiological role of the ubiquitin system in signal transduction and the regulation of several cell functions started to be appreciated and experimentally addressed. As with other principal mechanisms of signal transduction, such as phosphorylation or GTP hydrolysis, much of the information regarding the role of the ubiquitin system as a component of cell regulation and signaling cascades, was gained in studies of transformation and the control of cell growth. It seems, however, that ubiquitin-dependent proteolysis, and possibly other processes that are controlled by protein ubiquitination, play a role in many aspects of cellular function from the control of differentiation to intracelIular trafficking [1,3,4]. Here we will review some of the results that implicate ubiquitin-dependent proteolysis in the control of cell growth and that indicate how perturbations of ubiquitin-dependent degradation of oncogene and tumor suppressor gene products may contribute to cell transformation and oncogenesis.