A Phosphorylation Switch Modulates Configurational Codes in the Oncofetal IGF2BP RNA Binding Paralogs

The insulin like growth factor 2 mRNA binding proteins (IGF2BP1-3) are oncofetal RNA regulators that control translation, stability, and localization of several transcripts, yet display paralog specific functions despite high structural similarity. Each paralog contains six RNA binding domains (two RRMs and four KH domains) linked by intrinsically disordered segments. mTORC2 phosphorylates IGF2BP1 and IGF2BP3 at a single conserved serine within the disordered linker between the RRM2 and KH1 domains, a modification required for proper regulation of mRNA translational fate. Pairing site specific phosphoserine incorporation with structural and biophysical interrogations, we show that this phosphorylation acts as a configurational switch that reorganizes long range arrangements of RNA binding domains and linkers without altering the secondary structure, and with only modest effects on RNA binding affinity. Critically, pSer driven rearrangements occur both in the RNA free state and upon RNA engagement, and the resulting architectures differ markedly between IGF2BP1 and IGF2BP3 despite >70% sequence identity. These paralog specific, phosphorylation dependent configurational landscapes likely underlie differences in mRNA recognition modes and functional outcomes. Our work identifies a post translational mechanism that tunes IGF2BP paralog dynamics across free and RNA bound states to program target mRNA selection, processing, and translational fate.