The neurofibromatosis 2 protein product merlin selectively binds F-actin but not G-actin, and stabilizes the filaments through a lateral association

Abstract

The neurofibromatosis 2 protein product merlin, named for its relatedness to the ezrin, radixin and moesin (ERM) family of proteins, is a tumour suppressor whose absence results in the occurrence of multiple tumours of the nervous system, particularly schwannomas and meningiomas. Merlin's similarity to ERMs suggests that it might share functions, acting as a link between cytoskeletal components and the cell membrane. The N-terminus of merlin has strong sequence identity to the N-terminal actin-binding region of ezrin; here we describe in detail the merlin-actin interaction. Employing standard actin co-sedimentation assays, we have determined that merlin isoform 2 binds F-actin with an apparent binding constant of 3.6 microM and a stoichiometry of 1 mol of merlin per 11.5 mol of actin in filaments at saturation. Further, solid-phase binding assays reveal that merlin isoforms 1 and 2 bind actin filaments differentially, suggesting that the intramolecular interactions in isoform 1 might hinder its ability to bind actin. However, merlin does not bind G-actin. Studies of actin filament dynamics show that merlin slows filament disassembly with no influence on the assembly rate, indicating that merlin binds along actin filament lengths. This conclusion is supported by electron microscopy, which demonstrates that merlin binds periodically along cytoskeletal actin filaments. Comparison of these findings with those reported for ERM proteins reveal a distinct role for merlin in actin filament dynamics.

Merlin Expression in Secretory Meningiomas: Evidence of an NF2-independent Pathogenesis?

“…The COOH terminus of merlin is unique and lacks the conventional actin-binding region of the ERM proteins (merlin interacts with F-actin through its NH 2 terminus). 4,8,10,30,[32][33][34] Several experimental data demonstrated that the merlin overexpression results in a significant decrease in cell proliferation, reversion of Ras-induced transformation, and reduced tumor formation in nude mice. [30][31][32]35,36 The majority of the mutations identified in the NF2 gene results in a truncated protein.…”

“…4,8,10,30,[32][33][34] Several experimental data demonstrated that the merlin overexpression results in a significant decrease in cell proliferation, reversion of Ras-induced transformation, and reduced tumor formation in nude mice. [30][31][32]35,36 The majority of the mutations identified in the NF2 gene results in a truncated protein. 27,[37][38][39] Conflicting results have been reported with regard to the possible prognostic value of merlin in meningiomas.…”

Molecular networks controlling epithelial cell polarity in development

“…Interactions of cellcell adhesion complexes, e.g. catenin-cadherin with actin filaments either directly or through actin-associated proteins, such as a-actinin or members of the ezrin -radixinmoesin protein family have been described (James et al, 2001;Knudsen et al, 1995;Lallemand et al, 2003;Pujuguet et al, 2003;Rimm et al, 1995). Depletion of C. elegans ERM-1 function by RNAi abolishes specifically the assembly of actin microfilaments at the apical cortex of the gut cells.…”

Molecular mechanisms promoting the pathogenesis of Schwann cell neoplasms

“…As noted above, merlin lacks the carboxy terminal actin-binding domain that is present in ezrin, radixin and moesin. However, merlin may instead bind actin directly via sequences in its amino terminus [20,79] or indirectly through interactions with other ERM proteins [59,128,137], paxillin [49] or the cytoskeletal protein βII-spectrin [159]. Merlin also binds to and inhibits the action of other proteins directly involved in remodeling of the actin cytoskeleton such as N-WASP [117].…”