Nonmuscle myosin IIA dynamically guides regulatory light chain phosphorylation and assembly of nonmuscle myosin IIB

Nonmuscle myosin II minifilaments have emerged as central elements for force generation and mechanosensing by mammalian cells. Each minifilament can have a different composition and activity due to the existence of the three nonmuscle myosin II paralogs A, B and C and their respective phosphorylation pattern. We have used CRISPR/Cas9-based knockout cells, quantitative image analysis and mathematical modeling to dissect the dy-namic processes that control the formation and activity of heterotypic minifilaments and found a strong asym-metry between paralogs A and B. Loss of NM IIA completely abrogates regulatory light chain phosphorylation and reduces the level of assembled NM IIB. Activated NM IIB preferentially co-localizes with pre-formed NM IIA minifilaments and stabilizes the filament in a force-dependent mechanism. NM IIC is only weakly coupled to these processes. We conclude that NM IIA and B play clearly defined complementary roles during assembly of functional minifilaments. NM IIA is responsible for the formation of nascent pioneer minifilaments. NM IIB in-corporates into these and acts as a clutch that limits the force output to prevent excessive NM IIA activity. Together these two paralogs form a balanced system for regulated force generation.

Automated summary

Nonmuscle myosin II minifilaments are important for force generation and mechanosensing in mammalian cells. Different minifilaments have different compositions and activities, with a strong asymmetry between paralogs A and B. NM IIA is responsible for the formation of new minifilaments, while NM IIB acts as a regulator to prevent excessive activity. These two paralogs work together to form a balanced system.