Fig. 4: SSSCA1 belongs to a new family of Zn-binding proteins.

a SEC profiles of the N-terminal domain of SSSCA1 (black) and a set of three standard proteins (gray). The purified SSSCA1 N-terminal Auto_anti-p27 domain was subjected to SDS–PAGE and stained with Coomassie blue. b Crystal structure of the Auto_anti-p27 domain of human SSSCA1 at 2.3 Å resolution representing a new class of Zn-binding ribbon domain. Zinc ions are represented as orange spheres. Coordinating cysteine residues of each protomer are represented as sticks (wheat color). c Highly conserved cysteine residues are indicated as wheat colored sticks on the magnification of the Zn-binding site (orange sphere). The anomalous difference density map (orange map) is contoured at 5 σ and cover the Zn position. Lysine residues involved with ubiquitination are shown in gray and the conserved tyrosine cluster in dark cyan. An electron density map (2Fo-Fc, purple map) contoured at 1 σ is covering the tyrosine positions. The closest structure homologs of the core antiparallel β-sheet of the zinc-binding domain are two WW(Y) domains with conserved Trp and Tyr residues (sticks) highlighted: (d) The E3-ubiquitin-protein ligase SMURF1³⁸ (PDB ID 2LAZ) in yellow, and (e) the neuronal protein FE65³⁹ (PDB ID 2IDH) in magenta. f The C4 zinc-binding loops of the related Zn-binding ribbon domain 1 (ZNRD1) (gray) only have the position of the cysteines in common.