LAUSR

The formin INF2 polymerizes a calcium-activated cytoplasmic network of actin filaments, which we refer to as CIA (calcium-induced actin). CIA plays important roles in multiple cellular processes, including mitochondrial dynamics and vesicle transport. Here, we show that non-muscle myosin II (NMII) is activated within 60 sec of calcium stimulation and rapidly recruited to the CIA network. Knock-out of any individual NMII in U2OS cells affects the organization of the CIA network, as well as three downstream effects: ER-to-mitochondrial calcium transfer, mitochondrial Drp1 recruitment, and mitochondrial division. Interestingly, while NMIIC is the least abundant NMII in U2OS cells (>200-fold less than NMIIA and >10-fold less than NMIIB), its knock-out is equally deleterious to CIA. Based on these results, we propose that myosin II filaments containing all three NMII heavy chains exert organizational and contractile roles in the CIA network. In addition, NMIIA knock-out causes a significant decrease in myosin regulatory light chain levels, which might have additional effects.