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Vol. 13, Issue 5, 1512-1521, May 2002

and
*Department of Physiology, University of Wisconsin Medical School,
Madison, Wisconsin 53706; Dystrophin is widely thought to mechanically link the cortical
cytoskeleton with the muscle sarcolemma. Although the dystrophin homolog utrophin can functionally compensate for dystrophin in mice,
recent studies question whether utrophin can bind laterally along actin
filaments and anchor filaments to the sarcolemma. Herein, we have
expressed full-length recombinant utrophin and show that the purified
protein is fully soluble with a native molecular weight and molecular
dimensions indicative of monomers. We demonstrate that like dystrophin,
utrophin can form an extensive lateral association with actin filaments
and protect actin filaments from depolymerization in vitro. However,
utrophin binds laterally along actin filaments through contribution of
acidic spectrin-like repeats rather than the cluster of basic repeats
used by dystrophin. We also show that the defective linkage between
costameric actin filaments and the sarcolemma in dystrophin-deficient
mdx muscle is rescued by overexpression of utrophin. Our
results demonstrate that utrophin and dystrophin are functionally
interchangeable actin binding proteins, but that the molecular epitopes
important for filament binding differ between the two proteins. More
generally, our results raise the possibility that spectrin-like repeats
may enable some members of the plakin family of cytolinkers to
laterally bind and stabilize actin filaments.
Department of Human Anatomy and
Genetics, University of Oxford, Oxford OX1 3QX, United Kingdom; and
Department of Pathology and Laboratory Medicine, Robert
Wood Johnson Medical School, Piscataway, New Jersey 08854
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