Essential Signaling Function of Cytoplasmic NELFB Independent of RNA Polymerase II Pausing

Biochemical studies have established that the four-subunit negative elongation factor (NELF) complex mediates RNA polymerase II (Pol II) pausing at promoter proximal regions. Genetic ablation of individual NELF subunits destabilizes the entire NELF complex and causes lethality of cultured cells, leading to the prevailing concept that NELF-mediated Pol II pausing is essential for cell survival. Using separation-of-function mutations, we show here that NELFB's effects on cell proliferation can be uncoupled from its function in Pol II pausing. NELFB mutants localized in the cytoplasm and incapacitated in the NELF complex assembly still retain their ability to support cell proliferation and at least part of NELFB-dependent transcriptome. Furthermore, we demonstrate that cytoplasmic NELFB physically and functionally interacts with multiple pro-survival signaling kinases, most notably PI3K/AKT. Enforced activation of PI3K/AKT-related kinases largely rescues deficiency of NELFB-depleted cells in proliferation, but not Pol II pausing. Together, these data revise the current understanding of the growth impact of Pol II pausing and underscore multiplicity of the biological function of individual NELF subunits. Overall design: RNA libraries were prepared following the Illumina TruSeq stranded mRNA-seq sample preparation guide.