Phosphorylation-dependent contraction of actomyosin gels from amphibian eggs

Abstract

Actin networks or gels are a major structural component of the cytoplasm in most eukaryotic cells, especially those exhibiting motility^1,2. Cell-free extracts from a variety of cell types, including sea urchin eggs³ and Xenopus oocytes⁴, have been successfully used to examine the regulation of actin gelation and contraction. Although it has been shown that actin gel contraction is myosin-dependent (for review, see ref. 1), nothing is known of the role of myosin phosphorylation in this process. Indeed, much of the evidence suggesting that actomyosin-dependent movement in nonmuscle cells is regulated by myosin phosphorylation is provided by experiments showing that phosphorylation affects actin-activated ATPase activity^5,6. Here we report that the rate of actin gel contraction is increased, and several additional gels form and contract, in extracts from Xenopus laevis eggs prepared in conditions that promote protein phosphorylation. Since, in these conditions the 18,000-molecular weight (MW) light chain of myosin is the major phosphorylated component of the contracted gels, these results demonstrate a direct relationship between myosin phosphorylation and actin gel contraction. Evidence is also presented demonstrating that the recurrent waves of gelation and contraction occur in the absence of fluctuations in free-Ca²⁺ or pH.