Cloning and characterization of SCHIP-1, a novel protein interacting specifically with spliced isoforms and naturally occurring mutant NF2 proteins

The neurofibromatosis type 2 (NF2) protein, known as schwannomin or merlin, is a tumor suppressor involved in NF2-associated and sporadic schwannomas and meningiomas. It is closely related to the ezrin-radixin-moesin family m embers, implicated in linking membrane proteins to the cytoskeleton. The mo lecular mechanism allowing schwannomin to function as a tumor suppressor is unknown. In attempt to shed light on schwannomin function, we have identif ied a novel coiled-coil protein, SCHIP-1, that specifically associates with schwannomin in vitro and in vivo. Within its coiled-coil region, this prot ein is homologous to human FEZ proteins and the related Caenorhabditis eleg ans gene product UNC-76. Immunofluorescent staining of transiently transfec ted cells shows a partial colocalization of SCHIP-1 and schwannomin, beneat h the cytoplasmic membrane. Surprisingly, immunoprecipitation assays reveal that in a cellular context, association with SCHIP-1 can be observed only with some naturally occurring mutants of schwannomin, or a schwannomin spli ced isoform lacking exons 2 and 3, but not with the schwannomin isoform exh ibiting growth-suppressive activity. Our observations suggest that SCHIP-1 interaction with schwannomin is regulated by conformational changes in schw annomin, possibly induced by posttranstational modifications, alternative s plicing, or mutations.