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Vol. 18, Issue 4, 1507-1518, April 2007

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The Myosin IXb Motor Activity Targets the Myosin IXb RhoGAP Domain as Cargo to Sites of Actin Polymerization

Frank van den Boom^*, Heiko Düssmann^*,, Katharina Uhlenbrock^*, Marouan Abouhamed, and Martin Bähler

Institute for General Zoology and Genetics, Westfalian Wilhelms-University, 48149 Münster, Germany

Submitted September 5, 2006; Revised December 26, 2006; Accepted February 5, 2007
Monitoring Editor: Paul Forscher

Myosin IXb (Myo9b) is a single-headed processive myosin that exhibits Rho GTPase-activating protein (RhoGAP) activity in its tail region. Using live cell imaging, we determined that Myo9b is recruited to extending lamellipodia, ruffles, and filopodia, the regions of active actin polymerization. A functional motor domain was both necessary and sufficient for targeting Myo9b to these regions. The head domains of class IX myosins comprise a large insertion in loop2. Deletion of the large Myo9b head loop 2 insertion abrogated the enrichment in extending lamellipodia and ruffles, but enhanced significantly the enrichment at the tips of filopodia and retraction fibers. The enrichment in the tips of filopodia and retraction fibers depended on four lysine residues C-terminal to the loop 2 insertion and the tail region. Fluorescence recovery after photobleaching and photoactivation experiments in lamellipodia revealed that the dynamics of Myo9b was comparable to that of actin. The exchange rates depended on the Myo9b motor region and motor activity, and they were also dependent on the turnover of F-actin. These results demonstrate that Myo9b functions as a motorized RhoGAP molecule in regions of actin polymerization and identify Myo9b head sequences important for in vivo motor properties.

The online version of this article contains supplemental material at MBC Online (http://www.molbiolcell.org).

* These authors contributed equally to this work.

Present address: Department of Physiology and Medical Physics, Royal College of Surgeons in Ireland, York House, York Street, Dublin 2, Ireland.

Address correspondence to: Martin Bähler (baehler{at}uni-muenster.de)