TAB2 and TAB3 activate the Jun N-terminal kinase and nuclear factor-kappaB
pathways through the specific recognition of Lys 63-linked polyubiquitin chains
by its Npl4 zinc-finger (NZF) domain. Here we report crystal structures of the
TAB2 and TAB3 NZF domains in complex with Lys 63-linked diubiquitin at 1.18 and
1.40 A resolutions, respectively. Both NZF domains bind to the distal ubiquitin
through a conserved Thr-Phe dipeptide that has been shown to be important for
the interaction of the NZF domain of Npl4 with monoubiquitin. In contrast, a
surface specific to TAB2 and TAB3 binds the proximal ubiquitin. Both the distal
and proximal binding sites of the TAB2 and TAB3 NZF domains recognize the Ile
44-centred hydrophobic patch on ubiquitin but do not interact with the Lys
63-linked isopeptide bond. Mutagenesis experiments show that both binding sites
are required to enable binding of Lys 63-linked diubiquitin. We therefore
propose a mechanism for the recognition of Lys 63-linked polyubiquitin chains by
TAB2 and TAB3 NZF domains in which diubiquitin units are specifically recognized
by a single NZF domain.
