Chapter 17. Cell membranes and the cytoskeleton

P. Janmey
Experimental Medicine Division, Brigham and Women's Hospital,
Biomedical and Biological Sciences, Harvard Medical School,
Boston MA 02115, USA

1. Introduction

The interplay between the three-dimensional protein network called the cytoskeleton and the two-dimensional lipid bilayer which forms the cell membrane is a central feature of cell biology and a richly complex physical and chemical phenomenon. The complexity of the membrane/cytoskeleton boundary derives in part from the intricacy of the interface between two soft materials and in part from the number of distinct molecules and chemical interactions that occur at this interface and influence its physical properties and chemical composition. The importance of the field and the volume of contributions to it have motivated many reviews [1, 2] and a recent book devoted to this topic [3].

In the diversity of interactions between membrane constituents and components of the cytoskeleton, several themes reappear in different contexts.

• The binding of the major cytoskeletal fiber proteins themselves (except for some intermediate filaments) to phospholipids appears generally to be relatively weak and transient. Direct biochemical interactions of actin filaments, microtubules, and some types of intermediate filaments with purified lipids have been documented in vitro, but whether these interactions dominate the membrane/cytoskeletal interface is uncertain.
• Rather than linking cytoskeletal filaments directly to the lipid bilayer, intact cells assemble complexes of various proteins at points where the cytoskeleton attaches to the membrane. Some components in these linkages span the lipid bilayer, some penetrate into the cytoplasmic face of the bilayer, some bind preferentially to specific phospholipid headgroups, and others bind cytoskeletal filaments either directly to the lipid bilayer or indirectly to proteins bound to the membrane.
• In some cases the transmembrane proteins to which cytoskeletal filaments link are enzymes or ion transporters. The activity of these complexes is sometimes related to the extent of their attachment to the cytoskeleton and can alter the structure of both the membrane and the cytoskeletal network.
• Some types of cells possess two types of protein network linked to the membrane, a two dimensional lamina similar to that of the red cell and a three dimensional gel network linked either to the lipid bilayer or to the 2-D protein network.
• Motor molecules previously thought to produce motion primarily or exclusively between different protein assemblies, as in the sarcomere of the muscle, have now been shown to bind directly to purified lipids and to the plasma membrane of cells.
• Clustering of lipids and proteins within the membrane may be an important mechanism regulating their ability to produce biochemical and structural changes in the cell.
• Acidic phospholipids, particularly phosphoinositides appear to be especially important for interacting with cytoskeletal proteins, both by linking proteins to the bilayer and by biochemically regulating the activity of enzymes and cytoskeletal binding proteins.

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