MOLECULAR INSIGHTS INTO THE CELL PROLIFERATION REGULATOR SCI1: PHASE SEPARATION AND RH35 INTERACTION

SCI1 (Stigma/style Cell-cycle Inhibitor 1) protein has been described as a key regulator of floral cell proliferation. Although its transcript and protein expression in plants have been well characterized, including the tightly regulated protein degradation during mitosis, neither its structure nor its molecular function had been investigated in detail. Notably, we previously found that SCI1 has a high affinity interaction with DEAD-box RNA helicase 35 (RH35), a plant homolog of the human DDX41 protein and known component of the splicing machinery. However, the molecular basis of this interaction and whether SCI1 modulates RH35 function remain unknown. To address these questions, this work focuses on the biophysical and structural characterization of recombinant Nicotiana tabacum SCI1 and its interaction with RH35. SCI1 is a small protein predicted to be predominantly intrinsically disordered, an observation corroborated by circular dichroism spectroscopy. The only ordered domain is a conserved C-terminal region, designated as the “SCI1 motif.” In vitro, fluorescence microscopy experiments revealed that SCI1 undergoes phase separation, as does RH35. Additionally, SCI1 interacts with nucleic acids and nucleotides, such as DNA and ATP. How these properties regulate or contribute to the formation and dynamics of the SCI1–RH35 complex remains under investigation.

This work was supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES 88887.172952/2025-00) and by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP 2019/24774-1 and FAPESP 2026/04266-5).