Role of exon 2-encoded β-domain of the von Hippel-Lindau tumor suppressor protein

Abstract

Sporadic clear cell renal carcinomas frequently harbor inactivating mutations in exon 2 of the von Hippel-Lindau (VHL) tumor suppressor gene. Here, we examine the effect of the loss of exon 2-encoded β-domain function on VHL biochemical properties. Exon 2-encoded residues are required for VHL-mediated NEDD8 conjugation on cullin-2 and assembly with hypoxia-inducible factor α (HIFα) and fibronectin. These residues are not essential for VHL ability to assemble with elongin BC/cullin-2, to display E3 ubiquitin ligase activity in vitro and to confer energy-dependent nuclear import properties to a reporter protein. Localization studies in HIF-1α-null embryonic cells suggest that exon 2-encoded β-domain mediates transcription-dependent nuclear/cytoplasmic shuttling of VHL independently of assembly with HIF-1α and oxygen concentration. Exon 3-encoded α-helical domain is required for VHL complex formation with BC/cullin-2 and E3 ubiquitin ligase activity, for binding to HIF α/fibronectin, but this domain is not essential for transcription-dependent nuclear/cytoplasmic trafficking. VHL-/- renal carcinoma cells expressing β-domain mutants failed to produce an extracellular fibronectin matrix and to degrade HIFα, which accumulated exclusively in the nucleus of normoxic cells. These results demonstrate that exon 2-encoded residues are involved in two independent functions: substrate protein recognition and transcription-dependent nuclear/cytoplasmic trafficking. They also suggest that β-domain mutations inactivate VHL function differently than α-domain mutations, potentially providing an explanation for the relationship between different mutations of the VHL gene and clinical outcome.