Structure of the human Nac1 POZ domain

Abstract

Nac1 is a POZ-domain transcription factor that is involved in the self-renewal of embryonic stem cells. It is overexpressed in ovarian serous carcinoma and targeting the interactions of its POZ domain is a potential therapeutic strategy. Nac1 lacks a zinc-finger DNA-binding domain and thereby differs from most other POZ-domain transcription factors. Here, the crystal structure of the Nac1 POZ domain at 2.1 A resolution is reported. The Nac1 POZ domain crystallized as a dimer in which the interaction interfaces between subunits resemble those found in the POZ-zinc finger transcription factors. The organization of the Nac1 POZ-domain core resembles reported POZ-domain structures, whereas the C-terminus differs markedly. The C-terminal alpha-helix of the Nac1 POZ domain is shorter than that observed in most other POZ-domain transcription factors; variation in the organization of this region may be a general feature of POZ-domain structures.

Figure 1

Structure of the Nac1 POZ domain. (a) Sequence alignment of the Nac1, Miz-1, PLZF, BCL6 and LRF POZ domains. Sequences were aligned using ClustalW. The observed secondary-structure elements of the Nac1 POZ domain are indicated, with α-helices in orange and β-sheets in green. The residues of α6 in the Miz-1, PLZF, BCL6 and LRF POZ domains are shown in pale orange. (b) Ribbon representation of the Nac1 POZ-domain dimer. The position of the F98D mutation is shown with an asterisk. The secondary-structure elements of chain A are indicated, together with β1′ and β5′ of chain B.

Figure 2

Superposition of POZ-domain structures. (a) Stereo-image of POZ-domain C^α-atom superposition for Nac1 (red), BCL6 (green), PLZF (yellow) and LRF (blue). The accession codes of the PDB entries used for superposition were BCL6, 1r28; PLZF, 1buo; LRF, 2nn2. (b) Superposition of the BCL6 (pale green) and Nac1 (pale pink) POZ domains. The α5 and α6 regions of chain B are highlighted in green (BCL6) and red (Nac1).