Exploring the BICC1 Interactome in HEK293T Cells: Insights Into RNA-Mediated Protein Networks and Biomolecular Condensates.

Bicaudal C Homolog 1 (BICC1) is a conserved RNA-binding protein that, in mammals, has been primarily associated with polycystic kidney disease and renal organogenesis. However, its role in other disease contexts, including cancer, remains poorly understood. In this study, we characterized the BICC1 interactome, with emphasis on its dependence on RNA and the sterile alpha motif (SAM) domain, to identify novel biological processes associated with BICC1 function. Protein complexes were purified from HEK293T cells by co-immunoprecipitation and analyzed by mass spectrometry. Notably, co-immunoprecipitations performed in the presence of RNA yielded a larger number of interacting proteins, with 31 of 71 proteins (~43%) uniquely identified under RNA-preserved conditions, highlighting the critical role of RNA in mediating BICC1 protein-protein interactions. Enriched proteins were predominantly associated with mRNA splicing, the PRMT5 methylosome complex, and membraneless organelles, such as biomolecular condensates. Consistent with these findings, immunofluorescence assays performed on stressed cells revealed the co-localization of BICC1 with stress granule markers. Moreover, BICC1 interactions with PRMT5, STK38, PARP1, and IGF2BP1 were confirmed by immunoblotting.