Filament Binding Severing Proteins Create New Actin Ends

A second group of proteins, which bind to actin filaments, control the length of actin filaments by breaking them into shorter fragments and generating new filament ends for polymerization (Table 19-2). A valuable clue that led to the discovery of these severing proteins came from studies of amebas. Viscosity measurements and light-microscope observations demonstrated that during ameboid movement the cytosol flows forward in the center of the cell and then turns into a gel when it reaches the front end of the cell. As discussed later, this "sol to gel" transformation depends on the assembly of new actin filaments in the front part of a moving ameba and the disassembly of old actin filaments in the rear part. Because the actin concentration in a cell favors the formation of filaments, the breakdown of existing actin filaments and filament networks requires the assistance of severing proteins such as gelsolin and cofilin.

Severing proteins are thought to break an actin filament by stabilizing a change in the conformation of the subunit to which it binds; the resulting strain on the intersubunit bonds leads to its breakage. In support of this hypothesis are electron micrographs showing that an actin filament with bound cofilin is severely twisted. After a severing protein breaks a filament at one site, it remains bound at the (+) end of one of the resulting fragments, where it prevents the addition or exchange of actin subunits, an activity called capping. The (—) ends of fragments remain uncapped and are rapidly shortened. Thus severing promotes turnover of actin

TABLE 19-2 Some Cytosolic

Proteins That Control Actin Polymerization


Was this article helpful?

0 0
Your Heart and Nutrition

Your Heart and Nutrition

Prevention is better than a cure. Learn how to cherish your heart by taking the necessary means to keep it pumping healthily and steadily through your life.

Get My Free Ebook

Post a comment