A novel function for translation initiation factor eIF5

Interactions between proteins modulate essential functions of cells ensuring that they can grow and perform their tasks at the correct time and place. Each protein within a cell is like a piece within a complex machine. In a machine each part must be in the correct place and act at the correct time in concert with the other parts for the machine to function properly. The same is true for proteins within cells, but unlike most machines and their parts, proteins are continually renewed, move and can be modified by interaction with many other cell components to alter their function and/or location. Many proteins act together within complexes composed of proteins or with other cell components (lipids, DNA or small molecules for example) to coordinate a common function. One group of proteins relevant to this proposal is required make (or synthesize) all new proteins in each cell. These are called 'protein synthesis factors'. Understanding how protein synthesis factors function and interact with each other is therefore fundamental to understanding how proteins are made in all cells in all organisms. By improving our understanding how processes such as protein synthesis work in normal cells it can help scientists understand diseases in which this process is altered, or how infectious agents such as viruses are able to hijack plant, animal or human cells and cause infectious diseases. In work leading up to this proposal we have identified a novel complex between two protein synthesis factors known as eIF2 and eIF5. This has lead to a hypothesis for the function of this complex which we intend to assess here using a combination of biochemical experiments with purified proteins 'in the test-tube' and genetic experiments using yeast cells as a model simple cell system to understand the biological significance of this interaction for the mechanism and regulation of protein synthesis.

Over the past few years it has emerged that the translation initiation factor 5 (eIF5) plays a central role in the formation of pre-initiation factor complexes that lead to translation initiation in eukaryotic cells. Prior to this, eIF5 was believed to be a factor that was recruited late in the initiation pathway to function in initiator codon recognition and as a GTPase activating protein (GAP) to promote hydrolysis of GTP bound to a second translation factor-eIF2. The currently accepted view of translation is that eIF5 is recruited to eIF2-GTP-tRNAi ternary complexes (TC) and is necessary to promote/stabilize interactions among several translation factors including eIFs 2,3, 1 and 4G prior to its role as a GAP. We have obtained and present biochemical and genetic evidence that eIF5 interacts with eIF2 independently of the ternary complex form of eIF2 and propose that this is an eIF2-GDP/eIF5 complex. Extrapolating from these findings and those of others we propose that this complex represents an eIF2-GDP/eIF5 complex that is released from ribosomes upon GTP hydrolysis by eIF5. We propose here a series of biochemical and yeast genetic experiments to test different hypotheses concerning the function of this novel complex.