Growth control by TOR signaling pathways
Division, accumulation of mass (growth) and death are all fundamental aspects of cell behaviour. All three processes are highly regulated and the loss of this regulation can have dire medical consequences. We use the model eukaryote Saccharomyces cerevisiae (brewers’ yeast) to elucidate how growth is regulated in eukayotes. Knowledge of these pathways will be essential if we wish i) to develop therapeutics that target them when they are dysregulated in disease (e.g. in tumour cells); and ii) augment these pathways for economic gain (e.g. to improve agricultural yields).
Cell growth is influenced by both intrinsic signals - such as the position in the cell cycle, or landmark cues, like the site of the previous cell division; and also extrinsic signals - such as the presence of nutrients, growth factors and other cells. When nutrients and other appropriate growth stimuli are present, cells promote growth by stimulating the synthesis of macromolecules, and thereby increase in size and mass. Conversely, when conditions do not favour growth, for example, when nutrients are limited, cells restrain anabolic processes and enhance the turnover of mass through catabolic processes such as autophagy. Many cells also restrict mass accumulation to discrete loci. For example budding yeast grow in a spatially polarized manner due to deposition of mass at only one end of the cell. Consequently cell growth normally occurs under strict temporal and spatial constraints. In humans, when these constraints are removed, the resulting unrestrained growth leads to tumour-forming hamartoma syndromes amongst other ailments.
TOR controls growth: A conserved, atypical ser/thr protein kinase (it resembles phosphatidylinositol lipid kinases) known as the Target Of Rapamycin or TOR (mTOR in mammals) is now appreciated to play a central role in orchestrating diverse aspects of cell metabolism and physiology in response to many upstream stimuli ( Wullschleger et al, Cell 2006). These large (~280kDa) kinases were first identified genetically in S. cerevisiae as the molecular target of a natural product, rapamycin (Box 1). In eukaryote cells, rapamycin first interacts with a proline isomerase known as FKBP12 and this complex then binds and inhibits TOR activity. In 2002, we ( Loewith et al., Mol Cell 2002) made the important discovery that TOR proteins operate in two distinct multprotein complexes which we named TORC1 and TORC2 (mTORC1 and mTORC2 in mammalian cells). TORC1 (Box 2) and TORC2 (Box 3) are independently regulated downstream of environmental as well as cell-intrinsic cues, and, in turn, each complex independently regulates distinct aspects of eukaryote growth.