Origin and phylogeny of symmetry structure of protein assemblies-requirement from function or phyloggenic consequence?

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A symmetry structure of protein assemblies is sometimes found in the systems of photosynthesis, an energy conversion system and vision, a signal transduction system. Significance of such symmetry structure was investigated from the following two points ; (1) necessity for function and (phylogeny of proteins having different origins. The major points for analyzes were (1) interaction between prosthetic groups and (2) domain structure and/or module structure of proteins. Analyzes were carried out on photosynthetic reaction center (C_2 symmetry), antenna system (maimly C_3 symmetry), rhodopsin family (C_3 symmetry), and also pseudo C_3 symmetry in ATPase, the last of which might have a significance as well as the symmetry structure.In the case of photosynthetic antenna system, the symmetry structure is an easy way to increase a number of pigments for light energy collection (or absorption). However, it is very rare that this kind of structure can be assigned to a consequence for the expression of function. For example, phycobiliproteins in cyanobacteria and red algae contain two additional helices at the N terminus of a putative ancestral protein that is assigned to the globin family. Owing to these two helices, phycobiliproteins acquired the symmetry structure different from that of globin families and at the same time, an interaction between chromophores was realized, which was necessity for antennafunction.In the case of ATPase, breakdown of symmetry structure is shown to be necessity for the expression of function, which was proved by the recent analysis of the three-dimensional structure.In conclusion, the current subject is concerned with the structure-function relationship of the protein assemblies whose structural data are accumulating very rapidly, and thus it should be continued for further analyzes.