DesMarais Vera, Ichetovkin Ilia, Condeelis John, Hitchcock-DeGregori Sarah E
Department of Anatomy and Structural Biology, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461, USA.
J Cell Sci. 2002 Dec 1;115(Pt 23):4649-60. doi: 10.1242/jcs.00147.
Rapid polymerization of a network of short, branched actin filaments takes place at the leading edge of migrating cells, a compartment enriched in activators of actin polymerization such as the Arp2/3 complex and cofilin. Actin filaments elsewhere in the cell are long and unbranched. Results reported here show that the presence or absence of tropomyosin in these different actin-containing regions helps establish functionally distinct actin-containing compartments in the cell. Tropomyosin, an inhibitor of the Arp2/3 complex and cofilin function, was localized in relation to actin filaments, the Arp2/3 complex, and free barbed ends of actin filaments in MTLn3 cells, which rapidly extend flat lamellipodia following EGF stimulation. All tropomyosin isoforms examined using indirect immunofluorescence were relatively absent from the dynamic leading edge compartment, but did colocalize with actin structures deeper in the lamellipodium and in stress fibers. An in vitro light microscopy assay revealed that tropomyosin protects actin filaments from cofilin severing. The results suggest that tropomyosin-free actin filaments under the membrane can participate in rapid, dynamic processes that depend on interactions between the activities of the Arp2/3 complex and ADF/cofilin that tropomyosin inhibits elsewhere in the cell.
短的、分支状肌动蛋白丝网络的快速聚合发生在迁移细胞的前沿，这是一个富含肌动蛋白聚合激活剂（如Arp2/3复合体和丝切蛋白）的区域。细胞其他部位的肌动蛋白丝长且无分支。本文报道的结果表明，这些不同的含肌动蛋白区域中是否存在原肌球蛋白有助于在细胞中建立功能不同的含肌动蛋白区室。原肌球蛋白是Arp2/3复合体和丝切蛋白功能的抑制剂，在MTLn3细胞中，它相对于肌动蛋白丝、Arp2/3复合体和肌动蛋白丝的游离刺端定位，MTLn3细胞在表皮生长因子（EGF）刺激后会迅速伸展扁平的片状伪足。使用间接免疫荧光检测的所有原肌球蛋白异构体在动态的前沿区室中相对缺失，但确实与片状伪足深处和应力纤维中的肌动蛋白结构共定位。体外光学显微镜检测显示，原肌球蛋白可保护肌动蛋白丝不被丝切蛋白切断。结果表明，膜下无原肌球蛋白的肌动蛋白丝可参与快速、动态的过程，这些过程依赖于Arp2/3复合体和ADF/丝切蛋白活性之间的相互作用，而原肌球蛋白在细胞其他部位会抑制这种相互作用。
