Polyribosome targeting to microtubules: enrichment of specific mRNAs in a reconstituted microtubule preparation from sea urchin embryos

A subset of mRNAs, polyribosomes, and poly(A)-binding proteins copurify with microtubules from sea urchin embryos. Several lines of evidence indicate that the interaction of microtubules with ribosomes is specific: a distinct stalk-like structure appears to mediate their association; ribosomes bind to microtubules with a constant stoichiometry through several purification cycles; and the presence of ribosomes in these preparations depends on the presence of intact microtubules. Five specific mRNAs are enriched with the microtubule-bound ribosomes, indicating that translation of specific proteins may occur on the microtubule scaffolding in vivo. I N many embryos, the pattern of development that follows fertilization can be traced back to the localization of developmental determinants in the egg cytoplasm (Wilson, 1925; Davidson, 1986). The inheritance of a specific subset of mRNAs and proteins provides the macromolecular blueprints for the construction of diverse cellular forms with unique functions. In many cases, the compartmentalization of developmentally significant mRNAs is thought to occur through an association with the cytoskeleton. Similarly, in somatic cells, the cytoskeleton provides a network upon which individual mRNAs can be arranged in a three-dimensional pattern. The localized synthesis of specific proteins may have important consequences for the intrinsic regulation of cell fate and the specification of cellular domains such as axons and dendrites. All the major cytoskeletal structures, microtubules, microfilaments, and intermediate filaments, have been implicated in the targeting and transport of mRNA (reviewed by Jeffery, 1989; Fulton