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J. Biol. Chem., Vol. 278, Issue 5, 3089-3097, January 31, 2003

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Interactions between Nebulin-like Motifs and Thin Filament Regulatory Proteins^*

Ozgur Ogut, M. Moazzem Hossain, and Jian-Ping Jin

From the Department of Physiology and Biophysics, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106-4970

Nebulin (600-900 kDa) and nebulette (107-109 kDa) are two homologous thin filament-associated proteins in skeletal and cardiac muscles, respectively. Both proteins are capped with a unique region at the amino terminus as well as a serine-rich linker domain and SH3 domains at the COOH terminus. Their significant size difference is attributed to the length of the central region wherein both proteins are primarily composed of ~35 amino acid repeats termed nebulin-like repeats or motifs. These motifs are marked by a conserved SXXXY sequence and high affinity binding to F-actin. To further characterize the effects that nebulin-like proteins may have on the striated muscle thin filament, we have cloned, expressed, and purified a five-motif chicken nebulette fragment and tested its interaction with the thin filament regulatory proteins. Both tropomyosin and troponin T individually bound the nebulette fragment, although the affinity of this interaction was significantly increased when tropomyosin-troponin T was tested as a binary complex. The addition of troponin I to the tropomyosin-troponin T complex decreased the binding to the nebulette fragment, indicating an involvement of the conserved T2 region of troponin T in this interaction. F-actin cosedimentation demonstrated that the nebulette fragment was able to significantly increase the affinity of the tropomyosin-troponin assembly for F-actin. The relationships provide a means for nebulin-like motifs to participate in the allosteric regulation of striated muscle contraction.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AF440239.

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