Regulation of Microtubule Synthesis and Utilization during Early Embryonic Development of the Sea Urchin

Abstract

Microtubules deployed during early development of the sea urchin embryo are derived both from a preexisting pool of subunits present in the egg and from microtubule protein subunits synthesized in the embryo. Several aspects of microtubule protein synthesis and utilization are reviewed. Microtubule protein synthesis in early development utilizes oogenetic messenger RNA species. Translation of this mRNA is under regulation. Microtubule protein synthesis rises concomitantly with overall protein synthesis at fertilization, but rises at a relatively higher rate later in cleavage stages. Microtubule protein labeled with [³H]-leucine in early development is incorporated into cilia, indicating that newly synthesized protein enters the pool of subunits used in organelle assembly. The microtubule protein pool comprises about 1%of the soluble protein of the egg, and remains constant in size at least until the blastula stage. Direct pool size estimates are consistent with results of experiments on recruitment of microtubule protein subunits into the mitotic apparatus and into regenerating cilia. Soluble and particulate colchicine binding fractions, which have been reported from several systems, appear to be present in sea urchin embryos. The possible role of such fractions are discussed, as are aspects of the regulation of ciliary assembly.