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J. Cell Biol.,
Volume 142, Number 5, September 7, 1998 1181-1193
Department of Cell and Molecular Biology, Medical Nobel Institute, Karolinska Institutet, S-171 77, Stockholm, Sweden
Balbiani ring (BR) pre-mRNP particles reside
in the nuclei of salivary glands of the dipteran Chironomus tentans and carry the message for giant-sized salivary proteins. In the present study, we identify and
characterize a new protein component in the BR ribonucleoprotein (RNP) particles, designated hrp23. The protein with a molecular mass of 20 kD has a single
RNA-binding domain and a glycine-arginine-serine-rich
auxiliary domain. As shown by immunoelectron microscopy, the hrp23 protein is added to the BR transcript
concomitant with transcription, is still 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 translocating through the central channel of the complex. Thus, hrp23 is released just before or at the binding
of the particles to the nuclear pore complex. It is noted
that hrp23 behaves differently from two other BR RNP
proteins earlier studied: hrp36 and hrp45. These proteins
both 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 flow pattern, probably related to the particular role
of the protein in gene expression.
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