Chapter 19. Cation-induced vesicle fusion modulated by polymers and proteins
K. Arnold
Institute of Medical Physics and Biophysics, Department of Medicine,
University of Leipzig, Germany
1. Introduction
Membrane fusion is an important cell-physiological event that occurs in various intra- and intercellular processes, such as exocytosis, endocytosis, membrane genesis, viral infection, and fertilization. Hundreds of fusion events can occur every minute in a living cell. The diversity of cellular fusion processes could indicate that very different mechanisms are used by the cell to realize the fusion of membranes. This view is also supported by the large number of different cellular components involved in the triggering and temporal and spatial control of fusion. For instance, Ca2+ is required for exocytosis processes but it is not necessary for virus-cell fusion where the fusion of some viruses is triggered by low pH. However, the biophysical investigations of membrane fusion processes suggested that some general molecular mechanisms appear in all fusion processes. It will be shown in this chapter that a hydrophobic contact between the fusing membranes has to be created in order to initiate fusion of membranes. Another general requirement for fusion is the occurrence of packing defects among lipid molecules at the point of fusion.
The physical theories of membrane stability are relatively well established. However, a full understanding of membrane fusion processes requires not only a description of membrane stability but particularly of its transient instability. During the fusion event, drastic reorganizations in membrane structure must occur in order to allow the close approach and merging of membranes followed by the reconstitution of a new membrane. Because membranes serve as a relatively impermeable barrier, this function has to be maintained during the fusion process despite the transient destabilization. Therefore, research on membrane fusion is especially challenged to give an explanation of the interplay of membrane integrity and membrane destabilization. It's not surprising that investigations of fusion mechanisms have fertilized the development of theories of membrane stability, e.g., in respect to the role of the hydrophilicity and hydrophobicity of the membrane surface.
Studies of molecular mechanisms of membrane fusion processes have concentrated on areas that are relatively accessible to experimentation. At present virus-cell fusion represents the only example of a biological fusion where the molecular components involved in the fusion event are relatively well known. In an attempt to gain an understanding of the complex processes of cellular and subcellular fusion processes, much biophysical work has been devoted to studies of relatively simple model membranes such as lipid vesicles. As recognized in almost all fields of membrane research, both approaches are necessary for an understanding of the very complex processes of membrane fusion, the study of the fusion of real biological systems as well as of model systems. The last of these efforts is well covered by recent reviews [1-6].
This chapter has its focus on applications of phospholipid vesicles for the elucidation of molecular mechanisms of membrane fusion. Rather than presenting a complete review of work on vesicle fusion, basic biophysical concepts are demonstrated for fusion processes induced by cations, polymers and cation-binding proteins. Biological fusion processes are briefly reviewed to define the components involved in fusion processes. Fluorescence techniques that are frequently used to monitor the fusion are described. Before the fusion of vesicles is discussed, the aggregation of vesicles is considered with emphasis on the realization of a close approach of the membranes, recognized as a requirement for fusion. Some current models of vesicle fusion are discussed in the last sections of this chapter.
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