Biomolecular condensation of cMLCK enables myosin motor phosphorylation in the heart

Abstract

The heart needs to adapt its output to the metabolic demands of the organism. Phosphorylation of the myosin motors by cardiac myosin light chain kinase (cMLCK) increases heart muscle contractile function, yet its regulation and mechanism of action have remained unclear. Here, we show that cMLCK undergoes liquid-liquid phase separation and forms biomolecular condensates associated with the sarcoplasmic reticulum of cardiac muscle cells. Condensates selectively enrich enzymatic cofactors and substrates, which increases the catalytic activity of cMLCK. Our study reveals that cMLCK is fine-tuned to work in the molecular environment of condensates, enabling physiologically relevant levels of cardiac myosin motor phosphorylation. These findings establish a condensate-based mechanism for the spatial and temporal regulation of cardiac thick filament contractile function.