Interaction of the hnRNA of amphibian oocytes with fibril-forming proteins.

A ribonucleoprotein fraction that contains most of the rapidly labelled hnRNA has been isolated from gently ruptured oocytes of Triturus cristatus. This fraction consists of large aggregates of ribonucleoprotein and has a high (30:1) ratio of protein to RNA. The labelled RNA is contained in ribonucleoprotein particles that have a density of 1.27 g/cm3 in Cs2SO4 gradients (1.39 g/cm3 after formaldehyde fixation in CSCl gradients). Evidence is presented that the particles are associated in vivo with a fibrillar protein network. When the ribonucleoprotein aggregates are treated with ribonuclease, high salt concentration and nonionic detergent, a fibrillar protein residue is produced which contains many species of protein but a few that have electrophoretic characteristics that are identical to major ribonucleoprotein particle proteins. Isolated labelled hnRNA has been shown to bind specifically polypeptides of molecular weight 60 000 and 54 000 that are found in both particle and fibril preparations. In binding assays in vitro, these polypeptides are found to interact with mRNA to a lesser extent and not with rRNA. The isolated 60 000-Mr and 54 000-Mr proteins have the dual ability of forming ribonucleoprotein 'particles' with hnRNA and of polymerizing to generate 10-nm fibrillar structures in the absence of RNA. The possible cellular functions of these proteins are discussed.