Abstract
Several nuclear proteins undergo condensation. The question remains often whether this property is coupled to a functional aspect of the protein in the nucleus. The basic helix-loop-helix (bHLH) FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT) integrates internal and external signals to control the amount of iron that is acquired in accordance with growth. The previously described C-terminal Ser271/272 allows FIT to form active complexes with subgroup Ib bHLH factors such as bHLH039. FIT has lower nuclear mobility than mutant FITmSS271AA, but this behavior has remained mechanistically and functionally obscure. Here, we show that FIT undergoes a light-inducible subnuclear partitioning into nuclear condensates that we termed FIT nuclear bodies (NBs). The characteristics of FIT NBs could be examined using a standardized FIT NB analysis procedure coupled with different types of quantitative and qualitative microscopy-based approaches. We found that FIT condensates were likely formed by liquid-liquid phase separation. FIT accumulated preferentially in FIT NBs versus nucleoplasm when engaged in protein complexes with itself and with bHLH039. FITmSS271AA, instead, localized to NBs with different dynamics. FIT colocalized with splicing and light signaling NB markers. Hence, the inducible highly dynamic FIT condensates link active transcription factor complexes with posttranscriptional regulation processes.
FIT undergoes light-induced condensation and localizes to NBs, likely via LLPS
Functionally relevant Ser271/272 defines an intrinsically disordered region and influences NB formation dynamics
NBs are preferential sites for FIT dimerization with FIT and bHLH039, dependent on Ser271/272
FIT NBs colocalize with NB markers related to splicing and light signalling
Competing Interest Statement
The authors have declared no competing interest.