Characterisation of the biflavonoid hinokiflavone as a pre-mRNA splicing modulator that inhibits SENP.

We have identified the plant biflavonoid hinokiflavone as an inhibitor of splicing in vitro and modulator of alternative splicing in cells. Chemical synthesis confirms hinokiflavone is the active molecule. Hinokiflavone inhibits splicing in vitro by blocking spliceosome assembly, preventing formation of the B complex. Cells treated with hinokiflavone show altered subnuclear organization specifically of splicing factors required for A complex formation, which relocalize together with SUMO1 and SUMO2 into enlarged nuclear speckles containing polyadenylated RNA. Hinokiflavone increases protein SUMOylation levels, both in in vitro splicing reactions and in cells. Hinokiflavone also inhibited a purified, E. coli expressed SUMO protease, SENP1, in vitro, indicating the increase in SUMOylated proteins results primarily from inhibition of de-SUMOylation. Using a quantitative proteomics assay we identified many SUMO2 sites whose levels increased in cells following hinokiflavone treatment, with the major targets including six proteins that are components of the U2 snRNP and required for A complex formation.

Protein arginine N-methyltransferase substrate preferences for different nη-substituted arginyl peptides.

Protein arginine N-methyltransferases (PRMTs) catalyze methyl-group transfer from S-adenosyl-L-methionine onto arginine residues in proteins. In this study, modifications were introduced at the guanidine moiety of a peptidyl arginine residue to investigate how changes to the PRMT substrate can modulate enzyme activity. We found that peptides bearing Nη-hydroxy or Nη-amino substituted arginine showed higher apparent kcat values than for the monomethylated substrate when using PRMT1, whereas this catalytic preference was not observed for PRMT4 and PRMT6. Methylation by compromised PRMT1 variants E153Q and D51N further supports the finding that the N-hydroxy substitution facilitates methyl transfer by tuning the reactivity of the guanidine moiety. In contrast, Nη-nitro and Nη-canavanine substituted substrates inhibit PRMT activity. These findings demonstrate that methylation of these PRMT substrates is dependent on the nature of the modification at the guanidine moiety.