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Originally published as MBC in Press, 10.1091/mbc.E08-01-0034 on September 17, 2008

Vol. 19, Issue 11, 5006-5018, November 2008

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Retrograde Flow and Myosin II Activity within the Leading Cell Edge Deliver F-Actin to the Lamella to Seed the Formation of Graded Polarity Actomyosin II Filament Bundles in Migrating Fibroblasts

Tom W. Anderson*, Andrew N. Vaughan*, and Louise P. Cramer*,{dagger}

*MRC-Laboratory Molecular Cell Biology, and {dagger}Department of Cell and Developmental Biology, Faculty of Life Science, University College London, London WC1E 6BT, United Kingdom

Submitted January 16, 2008; Revised July 28, 2008; Accepted September 5, 2008
Monitoring Editor: Paul Forscher

In migrating fibroblasts actomyosin II bundles are graded polarity (GP) bundles, a distinct organization to stress fibers. GP bundles are important for powering cell migration, yet have an unknown mechanism of formation. Electron microscopy and the fate of photobleached marks show actin filaments undergoing retrograde flow in filopodia, and the lamellipodium are structurally and dynamically linked with stationary GP bundles within the lamella. An individual filopodium initially protrudes, but then becomes separated from the tip of the lamellipodium and seeds the formation of a new GP bundle within the lamella. In individual live cells expressing both GFP-myosin II and RFP-actin, myosin II puncta localize to the base of an individual filopodium an average 28 s before the filopodium seeds the formation of a new GP bundle. Associated myosin II is stationary with respect to the substratum in new GP bundles. Inhibition of myosin II motor activity in live cells blocks appearance of new GP bundles in the lamella, without inhibition of cell protrusion in the same timescale. We conclude retrograde F-actin flow and myosin II activity within the leading cell edge delivers F-actin to the lamella to seed the formation of new GP bundles.

This was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E08-01-0034) on September 17, 2008.

Address correspondence to: Louise P. Cramer (l.cramer{at}ucl.ac.uk).

Abbreviations used: GP, graded polarity.




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