Immunocytochemical evidence for a stepwise assembly of Balbiani ring premessenger ribonucleoprotein particles

In the active Balbiani ring (BR) genes of the dipteran Chironomus tentans, the assembly of a specific pre-mRNP particle can be analyzed in situ, and the incorporation of hnRNP proteins into the nascent pre-mRNP can be directly visualized by immunoelectron microscopy. In the present study we have shown that hrp36, one of the major hnRNP proteins in Chironomus tentans, is continuously added to the nascent BR pre-mRNP particle throughout transcription and is localized along the entire BR RNP fiber. Interestingly, hrp36 becomes concealed during the structural transition that occurs during the formation of the mature BR RNP particle. This conclusion is based on the observation that hrp36 can be revealed by a monoclonal antibody during the initial assembly of the BR RNP fiber but becomes almost undetectable in the final packaging stage. The hrp36 protein, however, is not removed from the BR RNP particle since the ability of the monoclonal antibody to reveal hrp36 is restored by artificial relaxation of mature BR RNP particles. Another major hnRNP protein, hrp45, is also incorporated in a continuous manner into the nascent pre-mRNP fiber but remains accessible in mature BR RNP particles. Our results provide immunocytochemical evidence for drastic structural changes occurring in the final stage of BR pre-mRNP packaging, and suggest that different hnRNP proteins might be differently involved in the pre-mRNP assembly process.