Calmodulin binding to alpha 1-purothionin: solution binding and modeling of the complex

Calmodulin-binding domain of recombinant erythrocyte beta-adducin

Adducin is a 200-kDa heterodimeric protein of the cortical cytoskeleton of mammalian erythrocytes. Analogs are also abundant in brain and several other tissues. In vitro, adducin bundles F-actin and enhances the binding of spectrin to actin. Previous studies have established that the beta subunit of adducin binds calmodulin (CaM) in a Ca(2+)-dependent fashion with intermediate affinity (approximately 200 nM) and that this activity is destroyed by proteolysis. We have confirmed the trypsin sensitivity of CaM binding by beta-adducin and the existence of a 38- to 39-kDa protease-resistant core. Calpain I digestion generates a larger core fragment (49 kDa) that is also devoid of CaM-binding activity. Use of recombinant beta-adducin peptides generated from partial cDNA clones identified strong CaM-binding activity within the protease-sensitive domain in residues 425-461: KQQKEKTRWLNTPNTYLRVNVADEVQRNMGSPRPKTT in single-letter amino acid codes. This region of the molecule is highly conserved between mouse, rat, and human and shares structural features with CaM-binding sequences in other proteins. Multiple flanking PEST sequences (sequences rich in proline, glutamic acid, serine, and threonine residues that enhance proteolytic sensitivity) may contribute to the protease sensitivity of this region. Consensus sequences for phosphorylation by cAMP-dependent kinases and by protein kinase C (or CaM-dependent kinase) are also found within or near this CaM-binding domain. Collectively, these data suggest a structural basis for the regulation of adducin by Ca(2+)-dependent CaM binding and possibly by covalent phosphorylation and calpain I-mediated proteolysis as well.

Calmodulin-binding domain of recombinant erythrocyte beta-adducin

Adducin is a 200-kDa heterodimeric protein of the cortical cytoskeleton of mammalian erythrocytes. Analogs are also abundant in brain and several other tissues. In vitro, adducin bundles F-actin and enhances the binding of spectrin to actin. Previous studies have established that the beta subunit of adducin binds calmodulin (CaM) in a Ca(2+)-dependent fashion with intermediate affinity (approximately 200 nM) and that this activity is destroyed by proteolysis. We have confirmed the trypsin sensitivity of CaM binding by beta-adducin and the existence of a 38- to 39-kDa protease-resistant core. Calpain I digestion generates a larger core fragment (49 kDa) that is also devoid of CaM-binding activity. Use of recombinant beta-adducin peptides generated from partial cDNA clones identified strong CaM-binding activity within the protease-sensitive domain in residues 425-461: KQQKEKTRWLNTPNTYLRVNVADEVQRNMGSPRPKTT in single-letter amino acid codes. This region of the molecule is highly conserved between mouse, rat, and human and shares structural features with CaM-binding sequences in other proteins. Multiple flanking PEST sequences (sequences rich in proline, glutamic acid, serine, and threonine residues that enhance proteolytic sensitivity) may contribute to the protease sensitivity of this region. Consensus sequences for phosphorylation by cAMP-dependent kinases and by protein kinase C (or CaM-dependent kinase) are also found within or near this CaM-binding domain. Collectively, these data suggest a structural basis for the regulation of adducin by Ca(2+)-dependent CaM binding and possibly by covalent phosphorylation and calpain I-mediated proteolysis as well.

The linker of calmodulin--to helix or not to helix

The linker regions of the central helices of calmodulin and of troponin C are observed to be alpha-helices in crystal and in solution. However, these linkers are predicted to be non-helical by standard algorithms. Further, there is strong evidence that when calmodulin interacts with some of its targets this linker helix bends. The linker appears to be delicately balanced between helical and non-helical conformations. A review of this subject suggests that one can anticipate more unpredicted conformations for the central helices of the score of other proteins that have four EF-hand domains.