Normal membrane localization and actin association of the NF2 tumor suppressor protein are dependent on folding of its N-terminal domain.

The neurofibromatosis type 2 (NF2) tumor suppressor protein, known as schwannomin or merlin, is involved in linking membrane proteins to the cytoskeleton. Like the related ERM proteins, schwannomin has long been suspected of exhibiting a complex 3D organization caused by the association of different regions within the protein. Intramolecular interactions characterized to date are linking N-terminal sequences of the protein to C-terminal sequences. Here, we demonstrate, by a biochemical approach, the existence of a structured domain entirely contained within the N-terminal half of schwannomin. This structure, which is resistant to chymotryptic digestion, encompasses the FERM domain (residues 19-314), but excludes the 18 extreme N-terminal residues specific to schwannomin. The structure is disrupted by some, but not all, naturally occurring NF2 mutations. We investigated the significance of this structured domain in schwannomin cellular functions and found that normal schwannomin localization beneath the plasma membrane is directly dependent on proper folding of the N-terminal domain. In addition, folding of the N-terminal domain influences schwannomin interaction with actin through two novel actin-binding sites located in this region. These results suggest that loss of activity of several naturally occurring schwannomin mutants is due to disruption of the fold of the N-terminal domain, leading to loss of both membrane localization and actin association.

Kappa light chain-associated Fanconi's syndrome: molecular analysis of monoclonal immunoglobulin light chains from patients with and without intracellular crystals.

Plasma cell dyscrasias may be responsible for Fanconi's syndrome, due to the toxicity of a free monoclonal kappa light chain toward kidney proximal tubules. Eight cases of Fanconi's syndrome were analyzed. We compared the structures of VkappaI variability subgroup V domains from five cases of Fanconi's syndrome and one myeloma without renal involvement. Among Fanconi cases, four putative structures were obtained after molecular modeling by homology, and the other had previously been refined by X-ray crystallography. The complete sequences of one VkappaI, one VkappaIII and N-terminal sequences of two VkappaI light chains, from patients with different forms of Fanconi's syndrome, were compared with four previously studied sequences. All three kappa chains responsible for a 'classical' form with intralysosomal crystals and a low mass myeloma, were encoded by the LCO2/O12 germline gene and had an unusual non-polar residue exposed to the solvent in the CDR-L1 loop. Of both VkappaI light chains from patients with Fanconi's syndrome without intracellular crystals, one derived from LCO2/O12 and the other from LCO8/O18 gene. Another feature that could be related to non-crystallization was the absence of accessible side chains in the CDR-L3 loop which is known to be implicated in dimer formation.