Actin-bound structures of Wiskott-Aldrich syndrome protein (WASP)-homology domain 2 and the implications for filament assembly.
Wiskott-Aldrich syndrome protein (WASP)-homology domain 2 (WH2) is a small and widespread actin-binding motif. In the WASP family, WH2 plays a role in filament nucleation by Arp2/3 complex. Here we describe the crystal structures of complexes of actin with the WH2 domains of WASP, WASP-family verprolin homologous protein, and WASP-interacting protein. Despite low sequence identity, WH2 shares structural similarity with the N-terminal portion of the actin monomer-sequestering thymosin beta domain (Tbeta). We show that both domains inhibit nucleotide exchange by targeting the cleft between actin subdomains 1 and 3, a common binding site for many unrelated actin-binding proteins. Importantly, WH2 is significantly shorter than Tbeta but binds actin with approximately 10-fold higher affinity. WH2 lacks a C-terminal extension that in Tbeta4 becomes involved in monomer sequestration by interfering with intersubunit contacts in F-actin. Owing to their shorter length, WH2 domains connected in tandem by short linkers can coexist with intersubunit contacts in F-actin and are proposed to function in filament nucleation by lining up actin subunits along a filament strand. The WH2-central region of WASP-family proteins is proposed to function in an analogous way by forming a special class of tandem repeats whose function is to line up actin and Arp2 during Arp2/3 nucleation. The structures also suggest a mechanism for how profilin-binding Pro-rich sequences positioned N-terminal to WH2 could feed actin monomers directly to WH2, thereby playing a role in filament elongation.
Chereau
David
D
Boston Biomedical Research Institute, Watertown, MA 02472, USA.
Kerff
Frederic
F
Graceffa
Philip
P
Grabarek
Zenon
Z
Langsetmo
Knut
K
Dominguez
Roberto
R
D000199:Actins / Q000378:metabolism*; D000595:Amino Acid Sequence; D002151:Calorimetry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009711:Nucleotides / Q000378:metabolism; D011485:Protein Binding; D017386:Sequence Homology, Amino Acid; D013329:Structure-Activity Relationship; D051316:Wiskott-Aldrich Syndrome Protein / Q000737:chemistry* / Q000378:metabolism
D000199:Actins; D009711:Nucleotides; C495767:WAS protein, human; D051316:Wiskott-Aldrich Syndrome Protein