High content of a nuclear pore complex protein in cytoplasmic annulate lamellae of Xenopus oocytes

The Xenopus laevis oocyte and egg represent an established model system to study nucleocytoplasmic transport and the assembly of the nuclear envelope (NE) and its pore complexes (PC). PCs, however, are not restricted to the NE but are also known to occur in cytoplasmic annulate lamellae (AL) in a variety of cells, including the Xenopus oocyte. However, the proportion of PCs found in such AL relative to those located in the NE, is unknown. In this study we have analyzed and quantified cytoplasmic AL in the full-grown (stage VI) Xenopus oocyte by immunolocalization at the light and electron microscopic level. Moreover, we have developed a method to enrich AL from enucleated oocytes, and have quantified a PC marker protein, nucleoporin p62, in both cytoplasmic AL and the NE. For this purpose we have used a specific monoclonal antibody (A225) which recognizes an epitope localized between amino acids 251 and 268 of Xenopus p62. We show that the number of PCs and p62 molecules present in AL far exceeds that of the NE. The possible implications of these findings to nucleocytoplasmic transport and nuclear PC (NPC) assembly are discussed.