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The structural effects of endogenous and exogenous Ca2+/calmodulin on phosphorylase kinase

, : The structural effects of endogenous and exogenous Ca2+/calmodulin on phosphorylase kinase. Journal of Biological Chemistry 272(42): 26202-9

The activity of phosphorylase b kinase (PbK) is stimulated by Ca-2+ ions, which act through its endogenous calmodulin subunit (delta), and further stimulated by the Ca-2+-dependent binding of exogenous calmodulin (delta'). In contrast to their highly characterized effects on activity, little is known regarding the structural effects on the (alpha-beta-gamma-delta)-4 PbK holoenzyme induced by Ca-2+ and delta'/Ca-2+. We have used mono- and bifunctional chemical modifiers as conformational probes to compare how the two effectors influence the structure of the catalytic y subunit and the interactions among all of the subunits. As determined by reductive methylation and carboxymethylation, Ca-2+ increased the accessibility of the gamma subunit; it also increased the formation by phenylenedimaleimide of an alpha-gamma-gamma conjugate that is characteristic of activated conformations of PbK (Nadeau, O. W., Sacks, D. M., and Carlson, G. M. (1997) J. Biol. Chem. 272, 26196-26201); however, Ca-2+ also had structural effects that were clearly distinct from other activators. Moreover, similar structural effects of Ca-2+ were observed with PbK that had been activated by phosphorylation, consistent with the fact that such activation does not eliminate the catalytic dependence of the enzyme on Ca-2+. Our results suggest tiers of conformational transitions in the activation of PbK, with the most fundamental being induced by Ca-2+. Analysis of the various cross-linked conjugates formed in the presence of Ca-2+ by o-phenylenedimaleimide or m-maleimidobenzoyl-N-hydroxysuccinimide ester showed that the binding of Ca-2+ to the delta subunit triggers changes in the interactions among all subunits, including between protomers, indicating an extensive communication network throughout the PbK complex. Most of the structural effects of delta'/Ca-2+ were qualitatively similar to, but quantitatively greater than, the effects of Ca-2+ alone; but delta'/Ca-2+ also had distinct effects, especially involving cross-linking of the delta subunit.


PMID: 9334188

DOI: 10.1074/jbc.272.42.26202

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