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Calcium‐ and magnesium‐dependent interactions between calcium‐ and integrin‐binding protein and the integrin αIIb cytoplasmic domain
Author(s) -
Yamniuk Aaron P.,
Vogel Hans J.
Publication year - 2005
Publication title -
protein science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.353
H-Index - 175
eISSN - 1469-896X
pISSN - 0961-8368
DOI - 10.1110/ps.041312805
Subject(s) - biophysics , isothermal titration calorimetry , cytoplasm , chemistry , calcium , binding site , protein structure , plasma protein binding , biochemistry , integrin , calcium binding protein , divalent , biology , receptor , organic chemistry
Calcium‐ and integrin‐binding protein (CIB) is a small EF‐hand calcium‐binding protein that is involved in hemostasis through its interaction with the αIIb cytoplasmic domain of integrinαIIbβ 3 . We have previously demonstrated that CIB lacks structural stability in the absence of divalent metal ions but that it acquires a well‐folded conformation upon addition of Ca 2+ or Mg 2+ . Here, we have used fluorescence spectroscopy, NMR spectroscopy, and isothermal titration calorimetry to demonstrate that both Ca 2+ ‐bound CIB (Ca 2+ ‐CIB) and the Mg 2+ ‐bound protein (Mg 2+ ‐CIB) bind with high affinity and through a similar mechanism to αIIb cytoplasmic domain peptides, but that metal‐free CIB (apo‐CIB) binds in a different manner. The interactions are thermodynamically distinct for Ca 2+ ‐CIB and Mg 2+ ‐CIB, but involve hydrophobic interactions in each case. Since the Mg 2+ concentration inside the cell is sufficient to saturate CIB at all times, our results imply that CIB would be capable of binding to the αIIb cytoplasmic domain independent of an intracellular Ca 2+ stimulus in vivo. This raises the question of whether CIB can act as a Ca 2+ sensor in αIIbβ 3 signaling or if other regulatory mechanisms such as fibrinogen‐induced conformational changes in αIIbβ 3 , post‐translational modifications, or the binding of other accessory proteins mediate the interactions between CIB and αIIbβ 3 . Differences in NMR spectra do suggest, however, that Ca 2+ ‐binding to the Mg 2+ ‐ CIB‐αIIb complex induces subtle structural changes that could further modulate the activity of αIIbβ 3 .