Actin bound to the heterogeneous nuclear ribonucleoprotein hrp36 is associated with Balbiani ring mRNA from the gene to polysomes

In the salivary glands of the dipteran Chironomus tentans, a specific messe nger ribonucleoprotein (mRNP) particle, the Balbiani ring (BR) granule, can be visualized during its assembly on the gene and during its nucleocytopla smic transport. We now show with immunoelectron microscopy that actin becom es associated with the BR particle concomitantly with transcription and is present in the particle in the nucleoplasm. DNase I affinity chromatography experiments with extracts from tissue culture cells indicate that both nuc lear and cytoplasmic actin are bound to the heterogeneous RNP (hnRNP) prote in hrp36. but not to the hnRNP proteins hrp23 and hrp45. The interaction is likely to be direct as purified actin binds to recombinant hrp36 in vitro. Furthermore, it is demonstrated by cross linking that nuclear as well as c ytoplasmic actin are bound to hrp36 in vivo. It is known that hrp36 is adde d cotranscriptionally along the BR mRNA molecule and accompanies the RNA th rough the nuclear pores and into polysomes, We conclude that actin is likel y to be bound to the BR transcript via hrp36 during the transfer of the mRN A from the gene all the way into polysomes.