Actin in Protein Synthesis and Protein Body Formation

Abstract

We review the recent evidence that plant cells, like animal cells, have mRNA associated with their cytoskeleton, and that much of this mRNA is present in polyribosomes. One of these cell types is the maize endosperm cell which contains storage proteins residing inside the lumen of the endoplasmic reticulum (ER). Data from both wild type and mutant maize endosperm support the proposition that the cytoskeleton is the scaffold upon which both polyribosomes (especially those synthesizing zein) and membranes adhere. While much of the data implies a major role for the actin cytoskeleton, additional roles are suggested for the microtubule system and for elongation factor, eEF1, which might help connect the two cytoskeleton networks. Results from rice endosperm have been especially beneficial to understanding the role of the cytoskeleton, insofar as rice has two entirely different kinds of protein bodies, the cereal-type and the legume-type. The former derive directly from the ER, store prolamines, and are closely associated with the cytoskeleton, while the latter arise from the coalescence of ER-derived vesicles, store glutelins, but are not associated with the cytoskeleton. The roles of the cytoskeleton appear to be in the segregation of specific mRNAs to specific sub-cellular locations, the enhanced translation of these mRNAs, and perhaps the accumulation of these water-insoluble storage proteins within the protein body.