Intrinsically disordered domain of transcription factor TCF-1 is required for T cell developmental fidelity

In development, pioneer transcription factors access silent chromatin to reveal lineage-specific gene programs. The structured DNA-binding domains of pioneer factors have been well characterized, but whether and how low-complexity intrinsically disordered regions (IDRs) affect chromatin and control cell fate is unclear. Here, we report deletion of an IDR of the pioneer factor TCF-1, termed “L1”, leads to an early developmental block in T cells. The few T cells that develop from progenitors expressing TCF-1 lacking L1 exhibit lineage infidelity distinct from the lineage diversion of TCF-1 deficient cells. Mechanistically, L1 is required for activation of T cell genes and de-repression of GATA2 driven genes, normally reserved to the mast cell and dendritic cell lineages. Underlying this lineage diversion, L1 mediates binding of TCF-1 to its earliest target genes which are subject to repression as T cells develop. These data suggest TCF-1’s intrinsically disordered N-terminus maintains T cell lineage fidelity.