Novel alternatively spliced isoforms of the neurofibromatosis type 2 tumorsuppressor are targeted to the nucleus and cytoplasmic granules

We cloned novel splice variants Mer150, Mer151 and Mer162 of the neurofibro matosis 2 (NF2) tumor suppressor, which demonstrate a tissue-specific and d evelopment-specific expression pattern. Isoform Mer150 is created by crypti c splicing from exon 8 to 14 and represents an N-terminal truncation of 259 residues. Mer151 is characterized by in-frame splicing out of several exon s and a modified C-terminus due to a frameshift in exons 13+14 and prematur e termination. Mer162 represents a head-to-tail isoform resulting from in-f rame skipping of exons 5-16, As a common feature, the alpha-helical domain and a variable proportion of the ERM homology domain are spliced out in the se isoforms, To investigate differences in subcellular localization, we exp ressed epitope-tagged cDNA constructs of the wild-type NF2 as well as of th e three alternatively spliced transcripts in NIH 3T3 cells by nuclear micro injection or lipid-mediated transfection, Subcellular localization of Mer15 1 in filopodia and ruffling membranes was similar to the wild-type NF2, Mer 151, however, was targeted to the nucleus, which was not observed for wild- type NF2, Mer150 or Mer162, A putative nuclear localization signal created by alternative splicing was identified in Mer151. In contrast to Mer151, Me r150 and Mer162 were not found in regions of the plasma membrane, but local ized to a granular intracellular compartment, The results suggest that the recently described actin-binding domain in exon 10, but not the presence or absence of exons 2+3, is relevant for subcellular targeting. Although the NF2 protein is known as a cytoskeletal linker, additional functions in a cy toplasmic compartment and in the nucleus may exist.