Nucleocytoplasmic transfer of the NF2 tumor suppressor protein merlin is regulated by exon 2 and a CRM1-dependent nuclear export signal in exon 15.
The neurofibromatosis 2 protein merlin is a classical tumor suppressor protein. Germline mutations predispose to the development of schwannomas, meningiomas and ependymomas. Merlin has been implicated in cellular migration and adhesion. This function is reflected in its subcellular localization at the plasma membrane and known interacting partners. Merlin has been regarded as an exception in not exerting a functional role within the nucleus as other tumor suppressors do. Here, we show that detection of wild-type protein in the nucleus is a rare event. However, splicing out of exon 2 leads to unrestricted entry into the nucleus. Skipping of adjacent exon 3 has no comparable effect ruling out an unspecific effect due to misfolding of the 4.1/JEF domain. Exon 2 functions as a cytoplasmic retention factor as it is able to confer sole cytoplasmic localization to a GFP fusion protein. Nuclear entry of merlin is thus regulated by alternative splicing within the 4.1/JEF domain and analogous to band 4.1 protein. Merlin's ability to enter the nucleus is complemented by a full nuclear-cytoplasmic shuttle protein with a functional Rev-type nuclear export sequence (NES) within exon 15 that facilitates export via the CRM1/exportin pathway. Deletion of this NES or treatment with the CRM1-specific inhibitor leptomycin B leads to overall nuclear accumulation of merlin isoforms missing exon 2. A cellular function different to the wild-type protein is implied for naturally occurring splice variants lacking exon 2. A putative effect of merlin as a transcriptional regulator and identification of nuclear binding partners remains to be elucidated.
Kressel
Michael
M
Institute of Anatomy I, Friedrich-Alexander University of Erlangen, 91054 Erlangen, Germany. michael.kressel@rzmail.uni-erlangen.de
Schmucker
Beatrice
B
D016475:3T3 Cells; D021581:Active Transport, Cell Nucleus / Q000502:physiology; D000595:Amino Acid Sequence; D000818:Animals; D002467:Cell Nucleus / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D005091:Exons; D006801:Humans; D028884:Karyopherins / Q000378:metabolism*; D051379:Mice; D018613:Microscopy, Confocal; D008969:Molecular Sequence Data; D025581:Neurofibromin 2 / Q000235:genetics / Q000378:metabolism*; D021382:Protein Sorting Signals; D021381:Protein Transport / Q000502:physiology*; D018160:Receptors, Cytoplasmic and Nuclear; D000097606:Exportin 1 Protein
D028884:Karyopherins; D025581:Neurofibromin 2; D021382:Protein Sorting Signals; D018160:Receptors, Cytoplasmic and Nuclear; D000097606:Exportin 1 Protein