Mutations to the ALS/FTD gene CCNF reveals elevated Lys48-linked ubiquitination by the SCF(cyclin F) complex

Approximately 10% of Amyotrophic lateral sclerosis (ALS) cases have a positive family history (familial ALS) and appear clinically indistinguishable from sporadic cases. ALS and frontotemporal dementia (FTD) are fatal neurodegenerative disorders that have common molecular and pathogenic characteristics; however, their biological mechanisms remain poorly understood. We have previously identified CCNF missense mutations in cohorts of familial and sporadic ALS and FTD cases [ref]. CCNF encodes cyclin F, a component of an E3 ubiquitin-protein ligase (SCFCyclin F) complex that is responsible for ubiquitinating proteins for degradation by the Ubiquitin-Proteasome System (UPS). The SCFCyclin F complex is essential for maintaining cellular homeostasis in cells; but mutations appear to lead to aberrant motor neuron development and neuron degeneration. We revealed elevated Lys48-specific ubiquitination of proteins in neuronal cells expressing mutant CCNFS621G compared to the CCNFWT control, which is consistent with increased Lys48-specific E3 ligase activity of cyclin FS621G (>1.3-fold). Different subsets of immunoprecipitated Lys48-ubiquitinated proteins were identified between CCNFWT and CCNFS621G cells indicating that transfected cyclin F contribute to the protein ubiquitination profile. These findings provide mechanistic insights into the effects of CCNF gene mutations on the function of the SCFcyclin F complex on neuronal proteostasis.