Kim Yongman, Ikeda Wataru, Nakanishi Hiroyuki, Tanaka Yoshinari, Takekuni Kyouji, Itoh Shinsuke, Monden Morito, Takai Yoshimi
Department of Molecular Biology and Biochemistry, Osaka University Graduate School of Medicine/Faculty of Medicine, Suita 565-0871, Japan.
Genes Cells. 2002 Apr;7(4):413-20. doi: 10.1046/j.1365-2443.2002.00524.x.
Frabin is an actin filament (F-actin)-binding protein with GDP/GTP exchange activity specific for Cdc42 small G protein. Expression of frabin forms filopodia-like microspikes through the direct activation of Cdc42, and lamellipodia through indirect activation of Rac small G protein. Frabin consists of the F-actin-binding domain (FAB), the Dbl homology domain (DH), the first pleckstrin homology domain (PH1), the FYVE-finger domain (FYVE), the second PH domain (PH2) from the N-terminus in this order. Although DH and PH1 show exchange activity, FAB, in addition to DH and PH1, is required for the formation of microspikes, whereas FYVE and PH2, in addition to DH and PH1, are required for the formation of lamellipodia.
Various truncated mutants of frabin were co-expressed with a dominant active mutant (DA) of Cdc42, Rac1DA, or full-length frabin in L fibroblasts. FAB was recruited to the Cdc42DA-formed filopodia-like microspikes. FAB and a fragment containing DH, PH1, FYVE and PH2 were recruited to the Rac1DA-formed membrane ruffles. Furthermore, each of these fragments served as a dominant negative mutant of frabin when co-expressed with full-length frabin, and inhibited the full-length frabin-formed morphological changes.
These results suggest that frabin recognizes a specific actin structure(s) through FAB and a specific membrane structure(s) through FAB and the region containing DH, PH1, FYVE and PH2. It is likely that frabin associates with the specific actin and membrane structures and activates Cdc42 and Rac in the vicinity of these structures, eventually leading to morphological changes.
Frabin是一种肌动蛋白丝（F-肌动蛋白）结合蛋白，具有对Cdc42小G蛋白特异的GDP/GTP交换活性。Frabin的表达通过直接激活Cdc42形成丝状伪足样微刺，通过间接激活Rac小G蛋白形成片状伪足。Frabin从N端起依次由F-肌动蛋白结合结构域（FAB）、双鸟苷酸交换因子同源结构域（DH）、第一个普列克底物蛋白同源结构域（PH1）、FYVE指结构域（FYVE）、第二个PH结构域（PH2）组成。虽然DH和PH1表现出交换活性，但除了DH和PH1外，微刺的形成还需要FAB，而除了DH和PH1外，片状伪足的形成还需要FYVE和PH2。
Frabin的各种截短突变体与Cdc42的显性活性突变体（DA）、Rac1DA或全长Frabin在L成纤维细胞中共表达。FAB被募集到由Cdc42DA形成的丝状伪足样微刺上。FAB以及包含DH、PH1、FYVE和PH2的片段被募集到由Rac1DA形成的膜皱褶上。此外，当这些片段中的每一个与全长Frabin共表达时，都作为Frabin的显性负性突变体，并抑制全长Frabin形成的形态变化。
这些结果表明，Frabin通过FAB识别特定的肌动蛋白结构，通过FAB以及包含DH、PH1、FYVE和PH2的区域识别特定的膜结构。Frabin可能与特定的肌动蛋白和膜结构结合，并在这些结构附近激活Cdc42和Rac，最终导致形态变化。
