| Research Abstract |
In many cases membraneous domain of membrane protein is a specific bundle of alpha-helices. What principle is responsible for such a specific structure formation? People tend to consider connecting loops between alpha-helices as a constraint to arrange the components. We, however, would like to regard that mutual recognition of alpha-helices would be most significant as a guideline for the formation of bundles in membrane. To prove the view whether correct or not, we attempted reconstruction of intramembrane structure of a membrane protein from fragments of original protein. Bacterioopsin, as a model, is a single polypeptide membrane protein composed of seven transmembrane helices A,B,・・・G,which are connected by loops, and retinal. Upon association with retinal it forms bacteriorhodopsin which has highly characteristic absorption spectrum and functions as a light-driven proton pump. In the present experiment, we have shown that bacteriorhodopsin chromophore can be reconstructed not only from bacterioopsin and retinal but also from independent alpha-helices, the residual part, and retinal, without loops connecting the components. In one case we afforded helices A and B as synthetic peptides and reconstruction was achieved from helices A,B,and the remaining part (C-G), which was derived from bacteriorhodopsin by chymotryptic digestion. In the other case reconstruction was successful with synthetic helices F and G, (A-E), which was a protease V8 fragment of bacteriorhodopsin, and retinal. These results clearly show that our idea on membrane protein structure formation as arising from mutual recognition of alpha-helices.
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