Journal
BIOSCIENCE BIOTECHNOLOGY AND BIOCHEMISTRY
Volume 67, Issue 8, Pages 1786-1796Publisher
TAYLOR & FRANCIS LTD
DOI: 10.1271/bbb.67.1786
Keywords
calpain; proteolysis; heterogeneous nuclear ribonucleoprotein K; Ca2+; yeast two-hybrid
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Calpain is a cytosolic modulator protease that modulates cellular functions in response to Ca2+. To identify in vivo substrates of calpain, yeast two-hybrid screening was done using the 5-EF-hand (penta-EF-hand; PEF) domain of the mu-calpain large subunit (domain IV), since several possible in vivo substrates for calpain have been previously reported to bind to the 5-EF-hand domains. Other than the regulatory subunit of calpain, which binds to the domain IV, heterogeneous nuclear ribonucleoproteins (hnRNP) K and R were identified, and shown to be proteolyzed by mu-calpain in vitro. When expressed in COS7 cells, hnRNP K and mu-calpain co-localized in the cytosol, and Ca2+-ionophore stimulation of the cells resulted in proteolysis of hnRNP K, indicating that hnRNP K is an in vivo substrate for calpain. Now, hnRNP K is considered to function as a scaffold protein for its binding proteins, such as PKCdelta and C/EBPbeta, which were reported to be calpain substrates, suggesting that hnRNP-K is a scaffold for calpain to proteolyze these proteins.
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