Heterogeneous Nuclear Ribonucleoprotein K Interacts with and Is Proteolyzed by Calpain in vivo

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  • KIMURA Eiichi
    <i>Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo</i>
  • ABE Keiko
    <i>Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo</i>
  • SUZUKI Koichi
    <i>Tokyo Metropolitan Institute of Gerontology</i>
  • SORIMACHI Hiroyuki
    <i>Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo</i> <i>CREST, JST</i>

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  • Heterogeneous Nuclear Ribonucleoprotein K Interacts with and Is Proteolyzed by Calpain<i>in vivo</i>

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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 μ-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 μ-calpain in vitro. When expressed in COS7 cells, hnRNP K and μ-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 PKCδ and C/EBPβ, which were reported to be calpain substrates, suggesting that hnRNP-K is a scaffold for calpain to proteolyze these proteins.<br>

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