Protein Kinase C Phosphomimetics Alter Thin Filament Ca(2+) Binding Properties

Adrenergic stimulation modulates cardiac function by altering the phosphorylation status of several cardiac proteins. The Troponin complex, which is the Ca(2+) sensor for cardiac contraction, is a hot spot for adrenergic phosphorylation. While the effect of β-adrenergic related PKA phosphorylation of troponin I at Ser23/24 is well established, the effects of α-adrenergic induced PKC phosphorylation on multiple sites of TnI (Ser43/45, Thr144) and TnT (Thr194, Ser198, Thr203 and Thr284) are much less clear. By utilizing an IAANS labeled fluorescent troponin C, [Image: see text], we systematically examined the site specific effects of PKC phosphomimetic mutants of TnI and TnT on TnC’s Ca(2+) binding properties in the Tn complex and reconstituted thin filament. The majority of the phosphomemetics had little effect on the Ca(2+) binding properties of the isolated Tn complex. However, when incorporated into the thin filament, the phosphomimetics typically altered thin filament Ca(2+) sensitivity in a way consistent with their respective effects on Ca(2+) sensitivity of skinned muscle preparations. The altered Ca(2+) sensitivity could be generally explained by a change in Ca(2+) dissociation rates. Within TnI, phosphomimetic Asp and Glu did not always behave similar, nor were Ala mutations (used to mimic non-phosphorylatable states) benign to Ca(2+) binding. Our results suggest that Troponin may act as a hub on the thin filament, sensing physiological stimuli to modulate the contractile performance of the heart.