The LIMD1 protein bridges an association between the prolyl hydroxylases and VHL to repress HIF-1 activity

There are three prolyl hydroxylases (PHD1, 2 and 3) that regulate the hypoxia-inducible factors (HIFs), the master transcriptional regulators that respond to changes in intracellular O-2 tension(1,2). In high O-2 tension (normoxia) the PHDs hydroxylate two conserved proline residues on HIF-1 alpha, which leads to binding of the von Hippel-Lindau (VHL) tumour suppressor, the recognition component of a ubiquiti-ligase complex, initiating HIF-1 alpha ubiquitylation and degradation(3-6). However, it is not known whether PHDs and VHL act separately to exert their enzymatic activities on HIF-1 alpha or as a multiprotein complex. Here we show that the tumour suppressor protein LIMD1 (LIM domain-containing protein) acts as a molecular scaffold, simultaneously binding the PHDs and VHL, thereby assembling a PHD-LIMD1-VHL protein complex and creating an enzymatic niche that enables efficient degradation of HIF-1 alpha. Depletion of endogenous LIMD1 increases HIF-1 alpha levels and transcriptional activity in both normoxia and hypoxia. Conversely, LIMD1 expression downregulates HIF-1 transcriptional activity in a manner depending on PHD and 26S proteasome activities. LIMD1 family member proteins Ajuba and WTIP also bind to VHL and PHDs 1 and 3, indicating that these LIM domain-containing proteins represent a previously unrecognized group of hypoxic regulators.