"DNA-->mRNA-->Protein"

this is a scheme of gene expression, so-called "Central Dogma" in molecular biology. However, the product here is a polypeptide in which amino acids are polymerized, not a functional protein. To get a mature protein, the polypeptide had to undergo several steps including modifications, foldings and so on. Moreover, proteins must be transported to the places where they actually function. Proteins are usually synthesized in cytosol, however, many of them function outside cytosol, especially outside cells and in the biomembranes. These proteins had to be translocated across or integrated into cellular membranes. Since the biomembranes (especially the plasma membranes) are the barrier between outside and inside the cells, the traverse of materials through the membranes is strictly restricted. For example, the small molecules such as ions and sugars, and even the water molecules are not allowed freely to translocate across the membranes. Therefore, some tactical mechanisms are required to translocate across and integrated into the biomembranes. The figure illustrates how membrane proteins are integrated into the ER membrane of eukaryotic cells (lower) and cytoplasmic membranes of bacterial cells (upper). While the precise names of the factor involved in the reactions are slightly different, the fundamental flows of the reactions are remarkably similar. Our purpose of the research is to clarify the detailed molecular mechanisms of the life phenomena that are conserved among all of the living things, by means of E. coli, a model organism. It is also used to be known that protein translocation across and integration into membranes are cold-sensitive processes. For these, our research is one of the most important projects to clarify the relationship between "temperature and life phenomena", a major purpose of the CRC Institute. We have succeeded in purification of factors involved in protein translocation and integration, and reconstitution of the reactions by means of the purified factors in vitro. During the processes, we have identified a novel glycolipid essential for protein integration. Although this factor is not proteinaceous, we have named it MPIase (Membrane Protein Integrase) after its enzyme-like function. We are now investigating the relationship between the structure and function of MPIase. We expect that plants and animals express an MPIase homolog because the mechanisms underlying protein integration into membranes are quite similar among all the living things. Especially, it is known that the mechanisms of protein transport in chloroplasts are quite similar to those in E. coli. These indicate that it is expected that modification of the chloroplast MPIase allows development of the cold-resistant plants.