Ikeda W, Nakanishi H, Tanaka Y, Tachibana K, Takai Y
Department of Molecular Biology and Biochemistry, Osaka University Graduate School of Medicine/Faculty of Medicine, Suita 565-0871, Japan.
Oncogene. 2001 Jun 14;20(27):3457-63. doi: 10.1038/sj.onc.1204463.
Frabin is a GDP/GTP exchange protein for Cdc42 with actin filament (F-actin)-binding activity. Cdc42 is a small GTP-binding protein that forms filopodia-like microspikes in a variety of cells. Expression of frabin indeed forms microspikes through at least activation of Cdc42 in MDCK cells and fibroblasts such as COS7, L, and NIH3T3 cells. However, the role of the F-actin-binding activity of frabin in the microspike formation remains unknown. We have examined here this role of frabin by expressing various frabin mutants, which have lost Cdc42-activating or F-actin-binding activity, with or without a dominant active mutant of Cdc42 in MDCK and COS7 cells. We show here that for the microspike formation, either of the Cdc42-activating and F- actin-binding activities of frabin alone is not sufficient and both the activities are necessary and that both the activities play a cooperative role in the microspike formation. The present results, together with the earlier finding that Cdc42 reorganizes the actin cytoskeleton at least through the N-WASP-Arp2/3 complex, suggest that frabin directly and indirectly reorganizes the actin cytoskeleton through its F-actin-binding and Cdc42-activating activities, respectively, in a cooperative manner, eventually leading to microspike formation.
Frabin是一种针对Cdc42的GDP/GTP交换蛋白，具有肌动蛋白丝（F-肌动蛋白）结合活性。Cdc42是一种小GTP结合蛋白，在多种细胞中形成丝状伪足样微刺。在MDCK细胞和成纤维细胞（如COS7、L和NIH3T3细胞）中，Frabin的表达确实至少通过激活Cdc42形成微刺。然而，Frabin的F-肌动蛋白结合活性在微刺形成中的作用仍不清楚。我们在此通过在MDCK和COS7细胞中表达各种已丧失Cdc42激活或F-肌动蛋白结合活性的Frabin突变体，以及有无Cdc42的显性活性突变体，来研究Frabin的这一作用。我们在此表明，对于微刺形成，Frabin单独的Cdc42激活和F-肌动蛋白结合活性中的任何一种都不足够，两种活性都是必需的，并且两种活性在微刺形成中起协同作用。目前的结果，连同早期发现Cdc42至少通过N-WASP-Arp2/3复合物重组肌动蛋白细胞骨架，表明Frabin分别通过其F-肌动蛋白结合和Cdc42激活活性以协同方式直接和间接重组肌动蛋白细胞骨架，最终导致微刺形成。
