Letter | Published:

Phosphorylation of platelet myosin increases actin-activated myosin ATPase activity



HUMAN platelet myosin, which is similar to other myosins isolated from non-muscle cells, has a molecular weight of 460,000 and is composed of two heavy chains (200,000) and two different light chains (20,000 and 15,000)1–3. The 20,000 light chain can be phosphorylated by a kinase endogenous to human platelets3. This enzyme, which has been isolated and partially purified4, transfers 32P from γ-32P-ATP to the 20,000 light chain of platelet myosin in the presence of Mg2+. So far, the biological significance of this phosphorylation has been unknown, but we report here that phosphorylation of platelet myosin results in an increase in the actin-activated myosin ATPase activity measured at low ionic strength. Dephosphorylation of phosphorylated myosin results in a decrease in the actin-activated ATPase activity.

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