Structure and function of the interacting domains of Spire and Fmn-family formins

Proc Natl Acad Sci U S A. 2011 Jul 19;108(29):11884-9. doi: 10.1073/pnas.1105703108. Epub 2011 Jul 5.


Evidence for cooperation between actin nucleators is growing. The WH2-containing nucleator Spire and the formin Cappuccino interact directly, and both are essential for assembly of an actin mesh during Drosophila oogenesis. Their interaction requires the kinase noncatalytic C-lobe domain (KIND) domain of Spire and the C-terminal tail of the formin. Here we describe the crystal structure of the KIND domain of human Spir1 alone and in complex with the tail of Fmn2, a mammalian ortholog of Cappuccino. The KIND domain is structurally similar to the C-lobe of protein kinases. The Fmn2 tail is coordinated in an acidic cleft at the base of the domain that appears to have evolved via deletion of a helix from the canonical kinase fold. Our functional analysis of Cappuccino reveals an unexpected requirement for its tail in actin assembly. In addition, we find that the KIND/tail interaction blocks nucleation by Cappuccino and promotes its displacement from filament barbed ends providing insight into possible modes of cooperation between Spire and Cappuccino.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Actins / metabolism*
  • Animals
  • Crystallization
  • Drosophila Proteins / chemistry
  • Drosophila Proteins / genetics
  • Drosophila Proteins / metabolism*
  • Drosophila melanogaster
  • Fluorescence Polarization
  • Humans
  • Microfilament Proteins / chemistry
  • Microfilament Proteins / genetics
  • Microfilament Proteins / metabolism*
  • Models, Molecular*
  • Nerve Tissue Proteins / chemistry*
  • Oogenesis / physiology*
  • Protein Conformation*
  • Protein Structure, Tertiary / genetics*


  • Actins
  • Drosophila Proteins
  • Microfilament Proteins
  • Nerve Tissue Proteins
  • capu protein, Drosophila
  • spir protein, Drosophila

Associated data

  • PDB/3R7G
  • PDB/3RBW