Structures of the Phactr1 RPEL domain and RPEL motif complexes with G-actin reveal the molecular basis for actin binding cooperativity

Structure. 2012 Nov 7;20(11):1960-70. doi: 10.1016/j.str.2012.08.031. Epub 2012 Oct 4.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Actins / metabolism*
  • Amino Acid Motifs
  • Amino Acid Sequence
  • Microfilament Proteins / chemistry*
  • Microfilament Proteins / metabolism
  • Models, Molecular
  • Molecular Sequence Data
  • Protein Binding
  • Sequence Homology, Amino Acid


  • Actins
  • Microfilament Proteins
  • Phactr-1 protein, human