Identification of Two RNA-Binding Proteins in Balbiani Ring Premessenger Ribonucleoprotein Granules and Presence of These Proteins in Specific Subsets of Heterogeneous Nuclear Ribonucleoprotein Particles

Abstract

Balbiani ring (BR) granules are premessenger ribonucleoprotein particles (RNPs) generated in giant chromosomal puffs, the BRs, in the larval salivary glands of the dipteran Chironomus tentans. Monoclonal antibodies were raised against nuclear proteins collected on a single-stranded-DNA–agarose affinity column, and two of them were used to identify RNA-binding proteins in BR granules. First, in Western blots (immu-noblots), one of the antibodies recognized a 36-kDa protein and the other recognized a 45-kDa protein. Second, both antibodies bound to the BRs in immunocytological experiments. It was shown in cross-linking experiments that the two proteins are associated with heterogeneous nuclear RNP (hnRNP) complexes extracted from C. tentans nuclei. By immunoelectron microscopy of isolated and partly unfolded BR RNPs, it was specifically demonstrated that the BR granules contain the two proteins and, in addition, that both proteins are distributed frequently along the RNP fiber of the particles. Thus, the 36- and 45-kDa proteins are likely to be abundant, RNA-binding proteins in the BR particles. To elucidate to what extent the two proteins are also present in other hnRNPs, we studied the binding of the antibodies to chromosomal puffs in general. It was observed that many puffs in addition to the BRs harbor the two proteins, but there are also puffs containing only one of the components, either the 36- or the 45-kDa protein. We conclude that the two proteins are not randomly bound to all hnRNPs but that each of them seems to be linked to a specific subset of the particles.