Cdc42 induces filopodia by promoting the formation of an IRSp53:Mena complex.
BACKGROUND
The Rho GTPases Rho, Rac, and Cdc42 regulate the organization of the actin cytoskeleton by interacting with multiple, distinct downstream effector proteins. Cdc42 controls the formation of actin bundle-containing filopodia at the cellular periphery. The molecular mechanism for this remains as yet unclear.


RESULTS
We report here that Cdc42 interacts with IRSp53/BAP2 alpha, an SH3 domain-containing scaffold protein, at a partial CRIB motif and that an N-terminal fragment of IRSp53 binds, via an intramolecular interaction, to the CRIB motif-containing central region. Overexpression of IRSp53 in fibroblasts leads to the formation of filopodia, and both this and Cdc42-induced filopodia are inhibited by expression of the N-terminal IRSp53 fragment. Using affinity chromatography, we have identified Mena, an Ena/VASP family member, as interacting with the SH3 domain of IRSp53. Mena and IRSp53 act synergistically to promote filopodia formation.


CONCLUSIONS
We conclude that the interaction of Cdc42 with the partial CRIB motif of IRSp53 relieves an intramolecular, autoinhibitory interaction with the N terminus, allowing the recruitment of Mena to the IRSp53 SH3 domain. This IRSp53:Mena complex initiates actin filament assembly into filopodia.
Krugmann
S
S
MRC Laboratory for Molecular Cell Biology and Cell Biology Unit, CRC Oncogene and Signal Transduction Group, University College London, Gower Street, London WC1E 6BT, United Kingdom.
Jordens
I
I
Gevaert
K
K
Driessens
M
M
Vandekerckhove
J
J
Hall
A
A
D016475:3T3 Cells; D000199:Actins / Q000378:metabolism; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D002352:Carrier Proteins / Q000378:metabolism*; D006224:Cricetinae; D003598:Cytoskeletal Proteins; D006367:HeLa Cells; D006801:Humans; D051379:Mice; D008840:Microfilament Proteins; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism; D011485:Protein Binding; D011554:Pseudopodia / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism; D020798:Two-Hybrid System Techniques; D020764:cdc42 GTP-Binding Protein / Q000378:metabolism*
D000199:Actins; C119539:BAIAP2 protein, human; D002352:Carrier Proteins; D003598:Cytoskeletal Proteins; C102176:Enah protein, mouse; D008840:Microfilament Proteins; D009419:Nerve Tissue Proteins; D010446:Peptide Fragments; D011994:Recombinant Proteins; D020764:cdc42 GTP-Binding Protein