Abstract
The PAF complex (PAFC) coordinates transcription elongation and mRNA processing and its CDC73/parafibromin subunit functions as a tumour suppressor. The NF2/Merlin tumour suppressor functions both at the cell cortex and nucleus and is a key mediator of contact inhibition but the molecular mechanisms remain unclear. In this study we have used affinity proteomics to identify novel Merlin interacting proteins and show that Merlin forms a complex with multiple proteins involved in RNA processing including the PAFC and the CHD1 chromatin remodeller. Tumour-derived inactivating mutations in both Merlin and the CDC73 PAFC subunit mutually disrupt their interaction and growth suppression by Merlin requires CDC73. Merlin interacts with the PAFC in a cell density-dependent manner and we identify a role for FAT cadherins in regulating the Merlin-PAFC interaction. Our results suggest that in addition to its function within the Hippo pathway, Merlin is part of a tumour suppressor network regulated by cell-cell adhesion which coordinates post-initiation steps of the transcription cycle of genes mediating contact inhibition.
Highlights
Normal cells cease to proliferate when they come into contact with each other and assemble intercellular junctions
We identified here new proteins that co-purified with Merlin such as: CDC73, PAF1, CTR9, LEO1, and SKI8, all core components of the PAF complex (PAFC); CHD1, a chromatin remodeller implicated in elongation [40]; components of the U2 small ribonucleoproteins complex (U2 snRNP) of the spliceosome; TAT-SF1, an elongation factor linked to splicing [41] and PELP1, a protein that interacts with several transcriptional activators and chromatin-modifying proteins and has been linked to splicing [42, 43] (Fig 1A and S1A Fig)
Endogenous Merlin co-immunoprecipitated with the PAFC components as well as CHD1 and VPRBP in IOMM-Lee meningioma cells [45] (Fig 1C), confirming that these interactions take place at physiological levels of Merlin as well as in the absence of SV40 Large T antigen that is expressed in HEK293T cells [46]
Summary
Normal cells cease to proliferate when they come into contact with each other and assemble intercellular junctions. Contact-dependent inhibition of proliferation is essential for normal tissue homeostasis and its loss is one of the hallmarks of cancer [1, 2]. The molecular mechanisms underlying contact inhibition remain unclear but Merlin, the protein encoded by the NF2 tumour suppressor gene is known to be an important mediator [3]. Germline mutations in NF2 cause Neurofibromatosis type 2, a cancer syndrome characterized by the development of schwannomas, meningiomas and ependymomas. Somatic NF2 mutations are frequently found in these tumour types as well as at lower frequency in several others [3,4,5].
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