23 S6 Phosphorylation and Signal Transduction

Abstract

The process of growth plays a fundamental role in the life cycle of a cell, serving two distinct purposes depending on the cell type and the developmental stage of the organism (Thomas and Hall 1997; Su and O’Farrell 1998; Conlon and Raff 1999). Two scenarios of growth are readily distinguished. In the first, in response to mitogens, cells enter the cell cycle, grow during G₁ phase to a critical mass permissive for the transition to S phase, then continue to increase in size during G₂ before initiating mitosis and giving rise to two daughter cells (Pardee 1989). In the second, in response to an agonist, terminally differentiated cells increase in size to accommodate a specific demand without subsequent cell division (Laser et al. 1998). In addition, besides playing an important role in the normal life cycle of an organism, the process of cell growth is subverted in several pathological conditions, as in hyperplasia and cancer or diseases where cells grow abnormally (hypertrophy) or where they lose their capacity to grow (atrophy). An understanding at the molecular level of the processes that control cell growth is a fundamental biological issue and will be important for identifying targets for therapies aimed at fighting specific disease states. An obligatory requirement for cell growth is the activation of protein synthesis (Pardee 1989). The major products being generated by the protein synthetic apparatus are components of the translational machinery, notably ribosomal proteins. Indeed, it has been calculated that this process may consume as much as...