Dual function of calmodulin (δ) in phosphorylase kinase

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Abstract
The Ca2+-independent activity of fast skeletal muscle phosphorylase kinase, A0, can be reversibly stimulated by heparin more than 20-fold; concomitantly the Ca2+-dependent A2 activity is abolished completely. Heparin also drastically changes the aggregation state of the enzyme; aggregated species contain significantly less .delta. and show an about 5-fold higher A0 activity than the tetrameric form containing .delta. stoichiometrically. This suggests that .delta. has 2 functions in the phosphorylase kinase: an inhibitory one with respect to A0 and an activating one with respect to A2. The inhibition of A0 by Ca2+-free .delta. is released, i.e., Ao increases when this subunit dissociates from the holoenzyme. The maximally heparin-stimulated A0 activity, A0,hep, is enriched from a crude extract to the same degree and approximately with the same yield as the major activity, A2. The phosphorylase kinase is not eluted from DEAE-cellulose as a symmetrical bell-shaped protein peak. The peak fraction contains the activities A2 and A0,hep superimposed and yields a nearly homogeneous sedimentation boundary with an s20,w value of 25.5 S. The A0 yields a much broader elution profile showing a distinct maximum from the A2 activity which contains slower sedimenting species of 12.1 S, some tetrameric enzyme of 22.7 S and higher aggregated material. Over the whole profile the activity ratio A2/A0 decreases about 7-fold, whereas the ratio A2/A0,hep is constant on average. This shows that A0 is an intrinsic activity of phosphorylase kinase. The heparin-activated A0 activity or A0 itself in the presence of the phosphorylase phosphatase inhibitor, F-, can trigger a Ca2+-independent flash activation of phosphorylase in a protein-glycogen complex. Thus, A0 could be responsible for the conversion of phosphorylase b to a at 20 nM free Ca2+ in resting, hormone-stimulated, muscle.
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