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© The Rockefeller University Press, 0021-9525/1998//1181 $5.00
The Journal of Cell Biology, Volume 142, Number 5, , 1998 1181-1193

Articles

The hrp23 Protein in the Balbiani Ring Pre-mRNP Particles Is Released Just before or at the Binding of the Particles to the Nuclear Pore Complex



Xin Sun, Alla T. Alzhanova-Ericsson, Neus Visa, Youssef Aissouni, Jian Zhao, and Bertil Daneholt

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.

Key Words: hnRNP protein • nonshuttling proteins • nucleocytoplasmic transport • Balbiani rings • nucleolus

Abbreviations used in this paper: BR, Balbiani ring; hnRNP, heterogeneous ribonucleoprotein; NES, nuclear export signal; NPC, nuclear pore complex; pre-mRNA, pre-messenger RNA; RBD, RNA-binding domain; RNP, ribonucleoprotein.

Address all correspondence to Bertil Daneholt, Department of Cell and Molecular Biology, Medical Nobel Institute, Karolinska Institutet, S-171 77, Stockholm, Sweden. Tel.: +46-8-7287370. Fax: +46-8-313529. E-mail: bertil.daneholt{at}cmb.ki.se


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