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A more recent version of this article appeared on March 1, 2003
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Submitted on July 18, 2002
Revised on October 24, 2002
Accepted on November 22, 2002
1 Department of Physiology, University of Maryland, School of Medicine, 685 W. Baltimore St., Baltimore, MD 21201
2 Department of Physiology, University of Maryland, School of Medicine, 685 W. Baltimore St., Baltimore, MD 2120
3 Departments of Biochemistry and Molecular Biology, University of Maryland, School of Medicine, 685 W. Baltimore St., Baltimore, MD 2120
* Corresponding author. E-mail address: akons001{at}umaryland.edu.
The factors that organize the internal membranes of cells are still poorly understood. We have been addressing this question using striated muscle cells, which have regular arrays of membranes that associate with the contractile apparatus in stereotypic patterns. Here we examine links between contractile structures and the sarcoplasmic reticulum (SR) established by small ankyrin 1 (sAnk1), a ~17.5kDa integral protein of network SR. We used yeast-two-hybrid to identify obscurin, a giant, Rho-GEF protein, as the major cytoplasmic ligand for sAnk1. The binding of obscurin to the cytoplasmic sequence of sAnk1 is mediated by a sequence of obscurin that is C-terminal to its last Ig-like domain. Binding was confirmed in two in vitro assays. In one, GST-obscurin, bound to glutathione-matrix, specifically adsorbed native sAnk1 from muscle homogenates. In the second, MBP-obscurin bound recombinant GST-sAnk1 in nitrocellulose blots. Kinetic studies using surface plasmon resonance yielded a KD=130nM. Upon subcellular fractionation, obscurin was concentrated in the myofibrillar fraction, consistent with its identification as sarcomeric protein. Nevertheless, obscurin, like sAnk1, concentrated around Z-disks and M-lines of striated muscle. Our findings suggest that obscurin binds sAnk1, and are the first to document a specific and direct interaction between proteins of the sarcomere and the SR.
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