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Vol. 14, Issue 10, 4173-4180, October 2003

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Localization of a Class III Myosin to Filopodia Tips in Transfected HeLa Cells Requires an Actin-binding Site in its Tail Domain

F. Les Erickson ^*, Amoreena C. Corsa , Andréa C. Dosé , and Beth Burnside  

^* Department of Biological Sciences, Salisbury University, Salisbury, Maryland 21801; Department of Molecular and Cell Biology, University of California, Berkeley, California 94720

Submitted October 15, 2002; Revised June 9, 2003; Accepted June 11, 2003
Monitoring Editor: Thomas Pollard

Bass Myo3A, a class III myosin, was expressed in HeLa cells as a GFP fusion in order to study its cellular localization. GFP-Myo3A localized to the cytoplasm and to the tips of F-actin bundles in filopodia, a localization that is consistent with the observed concentration toward the distal ends of F-actin bundles in photoreceptor cells. A mutation in the motor active site resulted in a loss of filopodia localization, suggesting that Myo3A motor activity is required for filopodial tip localization. Deletion analyses showed that the NH₂-terminal kinase domain is not required but the CO₂H-terminal 22 amino acids of the Myo3A tail are required for filopodial localization. Expression of this tail fragment alone produced fluorescence associated with F-actin throughout the cytoplasm and filopodia and a recombinant tail fragment bound to F-actin in vitro. An actin-binding motif was identified within this tail fragment, and a mutation within this motif abolished both filopodia localization by Myo3A and F-actin binding by the tail fragment alone. Calmodulin localized to filopodial tips when coexpressed with Myo3A but not in the absence of Myo3A, an observation consistent with the previous proposal that class III myosins bind calmodulin and thereby localize it in certain cell types.

Abbreviations used: 3THDI, myosin III tail homology domain I; 3THDII, myosin III tail homology domain II; GFP, green fluorescent protein; F-actin, filamentous actin.

Corresponding author. E-mail address: burnside{at}socrates.berkeley.edu.

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