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Light-triggered myosin activation for probing dynamic cellular processes.

Myosin II is an ATPase motor protein essential for many cellular functions including cell migration[1] and division.[2] In nonmuscle cells, myosin modulates protrusions at the leading edge and promotes retraction at the trailing edge during migration,[3] while during cytokinesis, myosin is required for contraction of the cleavage furrow.[4] For nonmuscle myosin, these varied functions are regulated by phosphorylation of the associated myosin regulatory light chain (mRLC) protein at Ser19, which activates the myosin complex to promote myosin assembly, contractility, and stress fiber formation.[5] Upon phosphorylation of the mRLC at both Thr18 and Ser19, these activities are further enhanced.[6 ] The dramatic effects of phosphorylation can also be recapitulated in vitro. Specifically, myosin and the proteolytic derivative heavy meromyosin (HMM),[7] which contains only one-third of the C-terminal myosin tail, exhibit low in vitro activities when associated with the nonphosphorylated mRLC. Phosphorylation of Ser19 amplifies actin-activated ATPase activities 10 – 1000-fold[8] and leads to myosin-mediated actin translocation.[9]