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Deciphering the cellular pathway for transport of poly(A)-binding protein II

Published online by Cambridge University Press:  01 February 2000

ANGELO CALADO
Affiliation:
Institute of Histology and Embryology, Faculty of Medicine, University of Lisbon, 1649-028 Lisboa, Portugal
ULRIKE KUTAY
Affiliation:
Zentrum für Molekulare Biologie, Universität Heidelberg, Im Neuenheimer Feld 282, 69120 Heidelberg, Germany
UWE KÜHN
Affiliation:
Universität Halle, Institut für Biochemie, Kurt-Mothes-Strasse 3, 06120 Halle, Germany
ELMAR WAHLE
Affiliation:
Universität Halle, Institut für Biochemie, Kurt-Mothes-Strasse 3, 06120 Halle, Germany
MARIA CARMO-FONSECA
Affiliation:
Institute of Histology and Embryology, Faculty of Medicine, University of Lisbon, 1649-028 Lisboa, Portugal
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Abstract

Poly(A)-binding protein II (PABP2) is an abundant nuclear protein that binds with high affinity to nascent poly(A) tails, stimulating their extension and controlling their length. In the cytoplasm, a distinct protein (PABP1) binds to poly(A) tails and participates in mRNA translation and stability. How cytoplasmic PABP1 substitutes for nuclear PABP2 is still unknown. Here we report that PABP2 shuttles back and forth between nucleus and cytoplasm by a carrier-mediated mechanism. A potential novel type of nuclear localization signal exists at the C-terminus of the protein, a domain that is highly enriched in methylated arginines. PABP2 binds directly to transportin in a RanGTP-sensitive manner, suggesting an involvement of this transport receptor in mediating import of the protein into the nucleus. Although PABP2 is small enough to diffuse passively through the nuclear pores, protein fusion experiments reveal the existence of a facilitated export pathway. Accordingly, no transport of PABP2 to the cytoplasm occurs at 4 °C. In contrast, export of PABP2 continues in the absence of transcription, indicating that transport to the cytoplasm is independent of mRNA traffic. Thus, rather than leaving the nucleus as a passive passenger of mRNAs, the data suggest that PABP2 interacts with the nuclear export machinery and may therefore contribute to mRNA transport.

Type
Research Article
Copyright
2000 RNA Society

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