Biological Membranes: The Dynamics of their Organization

This review will be confined to a single topic-namely, the growing literature pointing to the apparent paradox between the specificity and stability of membrane structure on the one hand and its lability and dy­ namism on the other. I hope the exact meaning of this statement will be­ come apparent in the course of this review. The examination of membranes is a favorite topic these days: witness the many recent reviews dealing with membrane organization (25, 33, 96, 125, 126, 130,202). However, these re­ views have dealt mainly with the organization of membranes in space; what I propose to do (the reason for yet another review on membranes!) is to con­ centrate on the temporal and dynamic characteristics of their organization. Not that membrane organization is well understood; quite the contrary, the fact that there is a spate of review articles illustrates the dictum that many ideas, good and bad, can be formulated in the relative absence of concrete data. Many of the experimental observations are good and to the point, but the modes of membrane organization are proving hard nuts to crack. There is a lack of really definitive data, compared to that in other fields: in a 44page review on the assembly of biological structures (128), the topic of membranes occupies only 3 pages. Many models of membrane structure have been proposed (cf reviews above) including the oft cited model of Green and co-workers (93) and ending with Deamer's more recent formula­ tion (43). In light of the data presented in this article, models will have to be modified to take into account changes in membrane organization within the dimension of time. As a prologue, I need cite no specific references to point out the well known fact that morphologically-and cytologically-particular membrane types in cells are specific not only with regard to their localization in the cell but also with regard. to their protein composition. Thus, at a single point in time, a membrane can be characterized by its content of enzyme proteins (cf reviews above) and also by that of other proteins, as seen by comparing the gel electropherograms of proteins extracted from various membranes (65, 123, 186, 234). That the different cellular membranes have lipid sig­ natures as well (28, 67, 80) is not so well acknowledged. The specificity