Control of actin synthesis during the development of Acanthamoeba castellanii

Abstract

The regulation of actin synthesis was examined in encysting cells of Acanthamoeba castellanii. Proteins were radiolabeled in vivo at different stages of development. The separation of proteins by two-dimensional gel electrophoresis revealed the existence of at least three forms of actin, each differing slightly in isoelectric point. These three actin proteins were also detected if amoeba RNA was translated in a wheat germ system or a reticulocyte lysate system. Nearly no actin protein synthesis could be detected at late stages of development. RNA isolated from cysts, however, is capable of directing the synthesis of the three actin proteins in vitro. The relative rates of the in vivo and in vitro actin translation are reduced about twofold at earlier stages of development. During subsequent stages of development, the relative content of actin mRNA does not change, whereas the relative rate of actin synthesis is reduced about 20-fold at late stages of development. This result suggests there is some kind of late translational regulation of actin synthesis. The changes in the pattern of polysomes and in the subcellular distribution of mRNA indicate a pronounced reduction in the initiation of protein synthesis at late stages of development. Assuming that different mRNAs have different affinities for ribosome subunits or other factors in the initiation process, the preferential inhibition of actin synthesis might indicate a relatively low rate constant for chain initiation.