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Lookup NU author(s): Dr Heather Lamb,
Professor Alastair Hawkins
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GRP78 is a major protein regulated by the mammalian endoplasmic reticulum stress response, and up-regulation has been shown to be important in protecting cells from challenge with cytotoxic agents. GRP78 has ATPase activity, acts as a chaperone, and interacts specifically with other proteins, such as caspases, as part of a mechanism regulating apoptosis. GRP78 is also reported to have a possible role as a Ca2+ storage protein. In order to understand the potential biological effects of Ca2+ and ATP/ADP binding on the biology of GRP78, we have determined its ligand binding properties. We show here for the first time that GRP78 can bind Ca2+, ATP, and ADP, each with a 1:1 stoichiometry, and that the binding of cation and nucleotide is cooperative. These observations do not support the hypothesis that GRP78 is a dynamic Ca2+ storage protein. Furthermore, we demonstrate that whereas Mg2+ enhances GRP78 binding to ADP and ATP to the same extent, Ca2+ shows a differential enhancement. In the presence of Ca2+, the KD for ATP is lowered ∼11-fold, and the KD for ADP is lowered around 930-fold. The KD for Ca 2+ is lowered ∼40-fold in the presence of ATP and around 880-fold with ADP. These findings may explain the biological requirement for a nucleotide exchange factor to remove ADP from GRP78. Taken together, our data suggest that the Ca2+-binding property of GRP78 may be part of a signal transduction pathway that modulates complex interactions between GRP78, ATP/ADP, secretory proteins, and caspases, and this ultimately has important consequences for cell viability. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.
Author(s): Lamb HK, Mee C, Xu W, Liu L, Blond S, Cooper A, Charles IG, Hawkins AR
Publication type: Article
Publication status: Published
Journal: Journal of Biological Chemistry
ISSN (print): 0021-9258
ISSN (electronic): 1083-351X
Publisher: American Society for Biochemistry and Molecular Biology, Inc.
PubMed id: 16418174
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