Distinct overlapping sequences at the carboxy‐terminus of merlin regulate its tumour suppressor and morphogenic activity

The Neurofibromatosis 2 ( NF2) gene product merlin is a tumour suppressor, which in addition to inhibiting cell proliferation regulates cell morphology. The morphogenic properties of merlin may play a role in tumour suppression, as patient‐derived tumour cells demonstrate cytoskeletal abnormalities. However, it is still unclear how these functions are linked. The N‐terminal FERM ‐domain of merlin is highly homologous to the oncogenic protein ezrin, while the C‐termini are less conserved, suggesting that the opposite effect of the proteins on proliferation could be mediated by their distinct C‐terminal regions. In this study we characterize the role of the most C‐terminal residues of merlin in the regulation of proliferation, cytoskeletal organization, phosphorylation and intramolecular associations. In addition to the two full‐length merlin isoforms and truncating mutations found in patients, we focused on the evolutionally conserved C‐terminal residues 545‐547, also harbouring disease‐causing mutations. We demonstrate that merlin induces cell extensions, which result from impaired retraction of protrusions rather than from increased formation of filopodia. The residues 538‐568 were found particularly important for this morphogenic activity. The results further show that both merlin isoforms are able to equally inhibit proliferation, whereas C‐terminal mutants affecting residues 545‐547 are less effective in growth suppression. This study demonstrates that the C‐terminus contains distinct but overlapping functional domains important for regulation of the morphogenic activity, intramolecular associations and cell proliferation.