title: Structures of the Phactr1 RPEL domain and RPEL motif complexes with G-actin reveal the molecular basis for actin binding cooperativity.
abstract: The Phactr family of PP1-binding proteins and the myocardin-related transcription factor family of transcriptional coactivators contain regulatory domains comprising three copies of the RPEL motif, a G-actin binding element. We report the structure of a Phactr1 G-actin⋅RPEL domain complex. Three G-actins surround the crank-shaped RPEL domain forming a closed helical assembly. Their spatial relationship is identical to the RPEL-actins within the pentavalent MRTF G-actin⋅RPEL domain complex, suggesting that conserved cooperative interactions between actin⋅RPEL units organize the assembly. In the trivalent Phactr1 complex, each G-actin⋅RPEL unit makes secondary contacts with its downstream actin involving distinct RPEL residues. Similar secondary contacts are seen in G-actin⋅RPEL peptide crystals. Loss-of-secondary-contact mutations destabilize the Phactr1 G-actin⋅RPEL assembly. Furthermore, actin-mediated inhibition of Phactr1 nuclear import requires secondary contact residues in the Phactr1 N-terminal RPEL-N motif, suggesting that it involves interaction of RPEL-N with the C-terminal assembly. Secondary actin contacts by actin-bound RPEL motifs thus govern formation of multivalent actin⋅RPEL assemblies.
authors.affiliation: Structural Biology, CRUK London Research Institute, 44 Lincoln's Inn Fields, London WC2A 3LY, UK.
authors.forename: Stephane
authors.initials: S
authors.lastname: Mouilleron
authors.forename: Maria
authors.initials: M
authors.lastname: Wiezlak
authors.forename: Nicola
authors.initials: N
authors.lastname: O'Reilly
authors.forename: Richard
authors.initials: R
authors.lastname: Treisman
authors.forename: Neil Q
authors.initials: NQ
authors.lastname: McDonald
mesh_terms: D000199:Actins / Q000378:metabolism*; D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D008840:Microfilament Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011485:Protein Binding; D017386:Sequence Homology, Amino Acid
chemical_list: D000199:Actins; D008840:Microfilament Proteins; C570134:PHACTR1 protein, human