Distribution of actin filaments in a human fibroblast in culture. The cell was stained with an actin-specific antibody conjugated with the dye fluorescein. The antigen-antibody reaction, performed directly in the culture, results in localization of the actin. The actin filaments organized in linear bundles fluoresce and thus show their distribution in this nonmigrating cell. (Courtesy of Dr. Elias Lazarides.)
In addition to controlling the rate of polymerization of actin filaments, ABPs are responsible for their organization. For example, a number of proteins can modify or act on actin filaments to give them various specific characteristics:
• Actin-bundling proteins cross-link actin filaments into parallel arrays, creating actin filament bundles. An example of this modification occurs inside the microvillus, where actin filaments are cross-linked by the actin-bundling proteins fascin and fimbrin (see Fig. 4.3, page 92). This cross-linkage provides support and imparts rigidity to the microvilli.
• Actin filament-severing proteins sever long actin filaments into short fragments. An example of such proteins is gelsolin, a 90-kDa ABP that normally initiates actin polymerization but at high Ca2h concentrations causes severing of the actin filaments, converting an actin gel into a fluid state.
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