Membrane Structure and Function

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Abstract
An understanding of the biochemical basis of membrane function is an important goal of present day biology. In this paper, a biochemical ap- proach to the problem of the specific transport of sugars across the membrane of Escherichia coli is discussed. A new biochemical model for lactose transport system in this organism is presented, in which a specific membrane protein (M protein) plays the role of the sugar carrier. Experiments which have led to the discovery of such a protein, its specific labeling, and partial purification are briefly reviewed. Problems of membrane structure and function are of central interest in many important fields of biology today. The mechanism of transmission of the nerve impulse, the mode of interaction of cells with each other in the developing embryo, the mechanism of action of hormones such as insulin-all these apparently diverse fields of biology have in common the need for a much more complete understanding of membrane function for continued progress. The explication of membrane phenomena in biochemical terms, however, has proved to be an exceedingly refractory problem. For example, it has now been more than 40 years since the discovery of insulin. In this period, as a result of the work of hundreds of laboratories throughout the world, the chemistry of the hormone has been completely elucidated, and a great deal has been learned about the physiological framework in which the hormone acts. However, the fundamental biochemical effects of insulin on tissues remain still to be elucidated. As is well known, one of the principal theories of the mechanism of action of insulin is that it promotes in some undisclosed manner the passage of sugars across membranes. The present status of this theory has recently been reviewed by Levine (1), who was one of the first to bring it to attention. It is clear that a complete verification or rejection of this hypothesis depends upon an understanding of the fundamental mode in which sugars cross the cells of living membranes. It is indeed this aspect of membrane func- tion that has engaged the attention of our own laboratory, although we have chosen to study the problem in bacterial, rather than in mammalian systems. The membranes of living cells throughout Nature appear to be made of similar building blocks-lipid and protein. The lipid moieties of metabolically 347

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