Microfilaments and microfilamentassociated proteins

Reorganization of the microfilament network is involved in membrane blebbing during apoptosis. F-actin, present at the base of blebs during apoptosis (4, 7, 8), is necessary for bleb formation and the eventual formation of apoptotic bodies (9, 10). Whether actin itself is a direct target for caspases remains elusive. Several groups have proposed that actin is cleaved by caspases during apoptosis (11-14). However, Brown et al. (15) reported that in membrane-associated actin, a component of the cytoskeleton that links polymerized actin to the plasma membrane, cleavage occurred at a site devoid of a consensus motif for cleavage by caspases. Song et al. (16) reported protection of actin for proteolysis in vivo. Loss of contacts between neighbouring cells is caused by the disassembly of the cytoskeletal organization at the level of cell-cell adhesion sites. In fact, p-catenin, a known regulator of cell-cell adhesion, is proteolytically processed after induction of apoptosis. (3-Catenin cleavage by caspase-3 removes the amino- and carboxy-terminal regions of the protein. The resulting p-catenin product is unable to bind a-catenin which is responsible for actin filament binding and organization (17). Also, cleavage of gelsolin (18) may be a physiological effector of morphological changes during apoptosis. The function of gelsolin is to bind barbed ends of actin monomers to prevent monomer exchange. Expression of the gelsolin cleavage product in multiple cell types caused the cells to round up, detach from the substratum, and undergo nuclear fragmentation (18).

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