PDBsum entry 1t08

Cell adhesion/cell cycle

PDB id: 1t08

Contents

Protein chains

510 a.a.

46 a.a.

15 a.a.

Waters ×327

References listed in PDB file

Key reference

Title

Mechanism of phosphorylation-Dependent binding of apc to beta-Catenin and its role in beta-Catenin degradation.

Authors

N.C.Ha, T.Tonozuka, J.L.Stamos, H.J.Choi, W.I.Weis.

Ref.

Mol Cell, 2004, 15, 511-521. [DOI no: 10.1016/j.molcel.2004.08.010]

PubMed id

15327768

Abstract

The transcriptional coactivator beta-catenin mediates Wnt growth factor signaling. In the absence of a Wnt signal, casein kinase 1 (CK1) and glycogen synthase kinase-3beta (GSK-3beta) phosphorylate cytosolic beta-catenin, thereby flagging it for recognition and destruction by the ubiquitin/proteosome machinery. Phosphorylation occurs in a multiprotein complex that includes the kinases, beta-catenin, axin, and the Adenomatous Polyposis Coli (APC) protein. The role of APC in this process is poorly understood. CK1epsilon and GSK-3beta phosphorylate APC, which increases its affinity for beta-catenin. Crystal structures of phosphorylated and nonphosphorylated APC bound to beta-catenin reveal a phosphorylation-dependent binding motif generated by mutual priming of CK1 and GSK-3beta substrate sequences. Axin is shown to act as a scaffold for substrate phosphorylation by these kinases. Phosphorylated APC and axin bind to the same surface of, and compete directly for, beta-catenin. The structural and biochemical data suggest a novel model for how APC functions in beta-catenin degradation.

Figure 1.
Figure 1. Primary Structure of Axin and APC(A) Axin. The binding sites of partner proteins are indicated. RGS, regulator of G protein signaling-homologous domain; DIX, dishevelled and axin-interaction domain.(B) APC. The four β-catenin binding 15-mer repeats are shown as white boxes and labeled A–D, and the seven 20-mer repeats (labeled 1–7) are shown as black boxes. The three axin binding SAMP repeats are shown in gray. Olig, dimerization domain; arm, armadillo repeat domain; basic, basic region; dlg, Discs-large binding site.(C) Structure-based alignment of the human APC β-catenin binding sequences. Residue numbers are indicated. The standard alignments of the APC repeats are highlighted in yellow to show the shift in register revealed by the R3 complex structure. The “core homology region” is the basis of the standard alignments. The shaded region of R3 is observed in the nonphosphorylated structure. The five residues that constitute the motif for interaction with β-catenin arm repeats 5-9 are shown in red. The order of CK1 and GSK-3β phosphorylation is indicated on the alignment. The four serine residues visible in the structure are indicated in blue, and the two others that represent the priming phosphorylations are shown in green. The structure-based alignment of the E-cadherin sequence is indicated, including the locations of the three pSer residues (green boxes) observed in the crystal structure of its complex with β-catenin (Huber and Weis, 2001). The R6 sequence used in initial experiments is underlined.

Figure 2.
Figure 2. Structure of Unphosphorylated APC R3 Bound to β-Catenin(A) Ribbon diagram of the complex, with cylinders representing α helices. The β-catenin arm repeat domain is shown in gray (helices 1 and 2 of the arm repeat motif) and cyan (helix 3, which forms the groove) (Huber et al., 1997). The ICAT helical domain used for crystallization is shown in white. The superimposed crystal structures of the bound XTcf-3 (green), E-cadherin (yellow), APC-RA (blue), and APC-R3 (orange) ligands are shown.(B) Overlay of the extended peptide of R3 (orange) and E-cadherin (yellow) bound to β-catenin. Nitrogen and oxygen atoms are shown in blue and red, respectively. Residue numbers for cadherin are shown in parentheses.(C) Diagram of the interactions formed between APC R3 and β-catenin. APC residues are shown in ovals with key side chain interactions indicated. Hydrogen bonds and salt bridges are indicated by dashed lines, and nonpolar contacts by arcs.

The above figures are reprinted by permission from Cell Press: Mol Cell (2004, 15, 511-521) copyright 2004.