Tat-binding protein-1, a component of the 26S proteasome, contributes to the E3 ubiquitin ligase function of the von Hippel-Lindau protein

von Hippel Lindau (VHL) gene inactivation occurs in von Hippel Lindau (VHL) disease. The protein pVHL functions in a multi-subunit E3 ubiquitin ligase that targets the hypoxia-inducible transcription factor Hif1alpha for proteasomal degradation during normoxia. We establish that pVHL binds to Tat-binding protein-1 (TBP-1), a component of the 19S regulatory complex of the proteasome. TBP-1 associates with the beta-domain of pVHL and complexes with pVHL and Hif1 in vivo. Overexpression of TBP-1 promotes degradation of Hif1 in a pVHL-dependent manner that requires the ATPase domain of TBP-1. Blockade of TBP-1 expression by small interfering RNA ( siRNA) causes prolonged degradation kinetics of Hif1alpha. Several distinct mutations in exon 2 of VHL disrupt binding of pVHL to TBP-1. A pVHL mutant containing a P154L substitution coimmunoprecipitates with Hif1alpha, but not TBP-1, and does not promote degradation of Hif1alpha. Thus, the ability of pVHL to degrade Hif1alpha depends in part on its interaction with TBP-1 and suggests a new mechanism for Hif1alpha stabilization in some pVHL-deficient tumors.