Karyoskeletal proteins and the organization of the amphibian oocyte nucleus

Abstract

We have investigated the existence of structural components in the nucleus of the oocyte of Xenopus laevis and other amphibia that are insoluble in non-denaturing detergents and buffers of low and high ionic strength. These cells are particularly suitable for such studies as they have a high frequency of extrachromosomal amplified nucleoli and pore complexes of the nuclear envelope. Using biochemical and immunological techniques, we have shown these structures to contain only two major proteins. These are a polypeptide of Mr 145000, which is located in a meshwork of filaments specific to the nucleolar cortex, and certain nucleoplasmic bodies probably derived therefrom, and a polypeptide of Mr 68000, which is the predominant constituent of the lamina-pore complex structure. We show that the latter protein is related to, but not identical to, lamina proteins ('lamins') of somatic cells, indicating cell type-specificity of the expression of polypeptides of the lamin family. In addition, we describe a protein of Mr 180000, which is the major constituent of the dense fibrillar component of the nucleolus. This can be partially solubilized in buffers of moderately high ionic strength. We interpret proteins of this category as karyoskeletal components involved in the architectural organization of specific functional topology within the nucleus. In contrast to previous reports for other cell types we have found no other prominent high-salt-insoluble structures in the nuclear interior, indicating the absence of an extended internal nuclear matrix in this kind of nucleus.