Abstract

The hypoxia–inducible transcription factor (HIF) is a key component of the cellular adaptation mechanisms to hypoxic
conditions. HIFa subunits are degraded by prolyl-4-hydroxylase domain (PHD) enzyme-dependent prolyl-4-hydroxylation of
LxxLAP motifs that confer oxygen-dependent proteolytic degradation. Interestingly, only three non-HIFa proteins contain
two conserved LxxLAP motifs, including the putative RNA helicase with a zinc finger domain HELZ. However, HELZ
proteolytic regulation was found to be oxygen-independent, supporting the notion that a LxxLAP sequence motif alone is
not sufficient for oxygen-dependent protein destruction. Since biochemical pathways involving RNA often require RNA
helicases to modulate RNA structure and activity, we used luciferase reporter gene constructs and metabolic labeling to
demonstrate that HELZ overexpression activates global protein translation whereas RNA-interference mediated HELZ
suppression had the opposite effect. Although HELZ interacted with the poly(A)-binding protein (PABP) via its PAM2 motif,
PABP was dispensable for HELZ function in protein translation. Importantly, downregulation of HELZ reduced translational
initiation, resulting in the disassembly of polysomes, in a reduction of cell proliferation and hypophosphorylation of
ribosomal protein S6.