APPARENT CONTINUITY BETWEEN THE MESSENGER TRANSPORT ORGANIZER AND LATE RNA LOCALIZATION PATHWAYS DURING OOGENESIS IN XENOPUS

The localization of RNAs at the vegetal cortex in Xenopus oocytes is a complex process, involving at least two different pathways. The early , or messenger transport organizer (METRO), pathway, localizes RNAs su ch as Xlsirts, Xcat2 and Xwnt11 during stages 1 and 2 of oogenesis, wh ile the late pathway localizes RNAs such as Vg1 during stages 2-4. We demonstrate that the onset of Vg1 localization is characterized by its microtubule-independent binding to a subdomain of the endoplasmic ret iculum (ER). The formation of this unique ER structure is intimately a ssociated with the movement of the mitochondrial cloud toward the vege tal cortex. In addition, we demonstrate that the mitochondrial cloud c ontains a gamma-tubulin-positive structure that may function as a micr otubule organizing center for establishing microtubule tracks for Vg1 localization. These data, support, although they do not prove, a model in which the development of the late pathway machinery relies upon th e prior functioning of the early pathway. (C) 1998 Elsevier Science Ir eland Ltd.