Simona Darovic (1), Marija Srnko (1,2), Helena Motaln (1), Boris Rogelj (1,2,3)
(1) Department of Biotechnology, Jožef Stefan Institute, Ljubljana, Slovenia
(2) Biomedical Research Institute BRIS, Ljubljana, Slovenia
(3) Faculty of Chemistry and Chemical Technology, University of Ljubljana, Slovenia
RNA-binding proteins (RBPs) including FUS protein of the FET family are known to play a role in neurodegenerative diseases like frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS), which are characterized by progressive neuron loss. Aberrant nucleocytoplasmic localization of RNA-binding proteins, including FUS, was associated with neurotoxicity. FUS immunoreactive cytoplasmic inclusions are found in 3 % of familial ALS cases and in 10–15 % of FTLD cases. But the underlying pathological mechanisms of FUS mislocalization and aggregation in both diseases appear different. FUS is predominantly nuclear protein that possesses PY-type nuclear localization signal (NLS) at its extreme C-terminus. In ALS but not in FTLD, the mutations in the NLS of FUS are responsible for its impaired nuclear transport mediated by nuclear import receptor transportin 1 (TNPO1). We have recently reported on the phosphorylation of C-terminal tyrosine at position 526 in NLS of FUS that abolished FUS interaction with TNPO1 and potentially impaired transport of C-terminal FUS fragment into the nucleus. Since proteins with a molecular mass below 40 kDa can also passively enter/exit nucleus, here our aim was to elaborate on the phosphorylation state of Y526 in C-terminal fragment compared to full-length FUS and their exact nucleocytoplasmic localization. We show here that besides in sole C-terminal fragment of FUS, the Y526 undergoes phosphorylation also in full-length FUS in the cytoplasm. This cytoplasmic phosphorylation of Y526 may serve to fine tune the nucleocytoplasmic shuttling of FUS, to ensure that a small amount of FUS remains always present in the cytoplasm possibly for dendritic mRNA transport.
Keywords: ALS, FUS, phosphorylation, nucleocytoplasmic shuttling