Mechanism and Function of Formins in the Control of Actin Assembly

Supplementary Data

• Supplemental Animation

  The animation illustrates the tentative, working model of FH2-mediated processive capping described in the text. The model is derived from simple superpositions of the crystal structure of the Bni1 FH2 domain in complex with actin (Otomo et al. 2005) with the Holmes model of F-actin. The formin FH2 domain in complex with actin is modeled at the barbed-end of an actin filament as described in the text and in Fig. 7. The linker segments that connect the bridges are omitted in the model, as their conformations are not known. The animation sequence is as follows:

  1. The model rotates 360° to provide a three-dimensional perspective.
  2. The migrating FH2 bridge element (green) is shown oscillating between the open (down) and closed (up) configurations.
  3. The view zooms in to the barbed end, and an actin subunit is added at the exposed post site of the green bridge.
  4. As described in the text, subunit addition is proposed to induce the now-penultimate yellow actin subunit to adopt an F-actin orientation. The rocking motion highlights the rotation required to bring the yellow subunit from its initial strained orientation into the F-actin orientation. The two bridge elements and two bound actin subunits (now the yellow and tan actin subunits) are rotated as a rigid body. Note that this rotation maintains the post site interaction of the blue bridge, but breaks its knob site contact with the orange actin subunit. This effectively reverses the roles of the bridge elements—the blue bridge is now the loosely bound "migrating" bridge and the green bridge is the "bound" bridge as it is making dual (knob and post site) interactions with the yellow and tan actins, respectively.
  5. Three additional cycles of actin subunit addition are shown.
  6. Three cycles of subunit dissociation are shown.

  Credits:

  The animation was produced by Dr. Jun Lu, Dana-Farber Cancer Institute, using Pymol v0.99 (http://pymol.sourceforge.net) and Quicktime Pro.

  The FH2/actin contacts are derived from PDB ID 1Y64, Otomo et al. Nature 433:488-94 (2005).

  Download video file (MOV)