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Characterization of the unc-23 gene, an HSP-1 chaperone regulator, in Caenorhabditis elegans Rahmani Gorji, Poupak


Mutations in the unc-23 gene in the free living nematode, Caenorhabditis elegans, result in detachment and dystrophy of the anterior body wall musculature and a bent-head phenotype when grown on solid substrate (Waterston et al, 1980). This phenotype is not observed when animals are grown in liquid culture (Bullerjahn and Riddle, pers. comm.) Neither muscle cell growth and positioning nor myofilament assembly is affected in liquid grown unc-23 animals. Muscle cell attachment, however, is affected since a small amount of stress applied by rolling the animals under the coverslip results in detachment of the muscle cells from the hypodermis. The results of immunological staining with antibodies to basement membrane and hypodermal components suggest that the primary defect in unc-23 animals is located within the hypodermis. UNC-23 encodes a cytoplasmic protein with a domain similar to the mammalian chaperone regulator BAG-2 (BCL2-associated athanogene 2). Human BAG-2 and C. elegans UNC-23 share 40% amino acid identity and 62% similarity over the BAG domain and its upstream region. In mammals, the BAG family of chaperone regulators contain a conserved 45 amino acid region near their C termini (the B A G domain) that binds Hsp70/Hsc70 and control their chaperone activity (Takayama et al, 1999). UNC-23 is first expressed in only a few cells in 1.5 fold stage embryos. As animals develop and grow, UNC-23 expression expands and eventually includes several tissues. In adult animals, UNC-23 is expressed in the pharynx, the body wall muscle cells, the hypodermis, H cells, and the vulva. In body wall muscle cells, UNC-23 is localized in a pattern similar to that of the transmembrane protein, (3 integrin, suggesting that it is associated with the proteins that make up the dense bodies and M lines. In the hypodermis, UNC-23 is present throughout the tissue with the exception of the nuclei and is distributed in a pattern reminiscent of the intermediate filaments. A yeast two-hybrid screen identified the ATPase domain of the non-inducible heat shock protein 70, HSP-1, as an interacting partner with the COOH terminus of UNC- 23. This in vitro result was confirmed in vivo with the identification of two dominant suppressors of unc-23(e25) as alleles of the hsp-1 gene in C. elegans. The molecular lesions that result in HSP-1 suppressor activity are missense mutations located within the ATPase domain of the HSP-1 molecule. In vitro studies of BAG-2 in mammals suggest that this protein is a negative regulator of Hsc70. It is predicted that UNC-23 in C. elegans is also a negative regulator of the HSP-1 chaperone. The results from suppression analysis and the phenotypic characterization of unc- 23 mutants suggest that the UNC-23-HSP-1 interaction is required for the maintenance of hypodermal integrity and muscle cell attachment during mechanical stress in C. elegans.

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