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Vol. 14, Issue 3, 1138-1148, March 2003
and
Departments of *Physiology and
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 ~17.5-kDa 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 = 130 nM. On 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.
Biochemistry and Molecular Biology, University
of Maryland, School of Medicine, Baltimore, Maryland 21201
Corresponding author. E-mail address:
akons001{at}umaryland.edu.
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