THE HRP23 PROTEIN IN THE BALBIANI RING PRE-MRNP PARTICLES IS RELEASEDJUST BEFORE OR AT THE BINDING OF THE PARTICLES TO THE NUCLEAR-PORE COMPLEX

Balbiani ring (BR) pre-mRNP particles reside in the nuclei of salivary glands of the dipteran Chironomus tentans and carry the message for g iant-sized salivary proteins. In the present study, we identify and ch aracterize a new protein component in the BR ribonucleoprotein (RNP) p articles, designated hrp23. The protein with a molecular mass of 20 kD has a single RNA-binding domain and a glycine-arginine-serine-rich au xiliary domain. As shown by immunoelectron microscopy, the hrp23 prote in is added to the BR transcript concomitant with transcription, is st ill present in the BR particles in the nucleoplasm, but is absent from the BR particles that are bound to the nuclear pore complex or are tr anslocating through the central channel of the complex. Thus, hrp23 is released just before or at the binding of the particles to the nuclea r pore complex. It is noted that hrp23 behaves differently from two ot her BR RNP proteins earlier studied: hrp36 and hrp45. These proteins b oth reach the nuclear pore complex, and hrp36 even accompanies the RNA into the cytoplasm. It is concluded that each BR RNA-binding protein seems to have a specific now pattern, probably related to the particul ar role of the protein in gene expression.