Association of frabin with specific actin and membrane structures.
BACKGROUND
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.


RESULTS
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.


CONCLUSIONS
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.
Kim
Yongman
Y
Department of Molecular Biology and Biochemistry, Osaka University Graduate School of Medicine/Faculty of Medicine, Suita 565-0871, Japan.
Ikeda
Wataru
W
Nakanishi
Hiroyuki
H
Tanaka
Yoshinari
Y
Takekuni
Kyouji
K
Itoh
Shinsuke
S
Monden
Morito
M
Takai
Yoshimi
Y
D000199:Actins / Q000378:metabolism*; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D007739:L Cells; D051379:Mice; D008840:Microfilament Proteins / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D011554:Pseudopodia / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D020764:cdc42 GTP-Binding Protein / Q000378:metabolism
D000199:Actins; C485165:Fgd4 protein, mouse; D008840:Microfilament Proteins; D011994:Recombinant Proteins; D020764:cdc42 GTP-Binding Protein