PDBsum entry 2raw

Cell cycle

PDB id: 2raw

Contents

Protein chains

137 a.a.

56 a.a.

Metals

_ZN

References listed in PDB file

Key reference

Title

The mitotic regulator survivin binds as a monomer to its functional interactor borealin.

Authors

E.Bourhis, S.G.Hymowitz, A.G.Cochran.

Ref.

J Biol Chem, 2007, 282, 35018-35023. [DOI no: 10.1074/jbc.M706233200]

PubMed id

17881355

Abstract

Survivin is a member of the IAP (inhibitor of apoptosis) protein family, defined in part by the presence of a zinc-binding baculoviral inhibitory repeat (BIR) domain. Most BIR domains bind short sequences beginning with alanine, and in this manner, they recognize and block the action of key targets in apoptotic pathways. However, Survivin binds only very weakly to typical IAP ligands. Unique features of Survivin are the long C-terminal helix following the BIR domain and a short segment (linking the helix and BIR domains) that mediates Survivin homodimerization. Despite this detailed knowledge of the structure of Survivin itself, there is a current lack of understanding about how Survivin recognizes cellular binding partners, and consequently, many questions about Survivin function remain unanswered. We determined two co-crystal structures of Survivin and a minimal binding fragment from the chromosomal passenger protein Borealin, a well validated functional interactor. The interaction between Survivin and Borealin involves extensive packing between the long C-terminal helix of Survivin and a long Borealin helix. Surprisingly, an additional important interaction occurs between the Survivin homodimerization interface and a short segment of Borealin. This segment both structurally mimics and displaces one Survivin monomer. The relevance of this unexpected interaction was tested by mutagenesis of two key Borealin residues. Mutant Borealin introduced into HeLa cells failed to localize properly during mitosis and also caused mislocalization of other chromosomal passenger proteins. This suggests that the mutant is dominant-negative and confirms the functional importance of the interaction surface identified in the crystal structures.

Figure 1.
FIGURE 1. Defining the minimal fragment of Borealin necessary to bind Survivin and the Survivin interaction surface. A, aligned primary sequences of the Borealin N terminus. The yellow bars indicate sequences either strongly predicted (thicker bar) or weakly predicted (thinner bar) to form a coiled-coil structure. Details are shown in supplemental Fig. S1. Boundary residues for the truncation constructs are indicated above the sequences. B, summary of fragment copurification studies. Interactions were scored based on whether Borealin could be detected readily by Coomassie Blue staining (+), only by Western blotting ((+)), or by neither method (-) (see "Experimental Procedures"). n.d., not determined. C, inferred interaction surface (teal) of Borealin-(20-78) mapped onto the previously reported dimeric Survivin structure (Protein Data Bank code 1F3H) (4). The putative interaction surface spans the homodimer interface.

Figure 2.
FIGURE 2. Structure of the Survivin-Borealin complex. A, comparison of the overall architecture of the Survivin dimer and the 1:1 Survivin-Borealin complex. At left is a different view of the Survivin homodimer than that shown in Fig. 1C, with monomers shown in purple and blue (Protein Data Bank code 1F3H) (4). The 2.4-Å structure of Survivin-(1-142) and the Borealin fragment is shown at center, whereas the 3.3-Å Survivin-(1-120) structure is shown at right. Survivin is shown in white, and the Borealin fragment is shown in teal. B, close-up view of the Survivin dimer interface (far left) and the corresponding region of the Survivin-(1-142) complex (second from left). Subunits are colored as described for A. Note the rearrangement of Survivin side chains to accommodate the Borealin Trp^70 side chain. The overlay at second from right illustrates the backbone mimicry by Borealin of the displaced Survivin monomer, whereas the illustration at far right shows the Borealin-Survivin backbone overlay in greater detail. The sequences of the two segments shown at right are ^64ALREMNWLDY^73 (Borealin) and ^92QFEELTLGEF^101 (Survivin). Colors for the Survivin homodimer and the complex subunits are as shown in the left two illustrations. C, surface representation of Survivin showing the pocket that opens up to accommodate Borealin Trp^70. Borealin is shown as a yellow ribbon, and the side chain for Tyr^54 is also shown (see "Results"). The solvent-accessible surface of all Survivin residues with 4.2 Å of the Trp^70 side chain is colored blue. The side chains of these Survivin residues are shown as red sticks. D, comparison of the mode of Borealin recognition with the typical BIR domain peptide-binding site seen in IAP proteins. The BIR domains of Survivin and ML-IAP (Protein Data Bank code 1OXQ) (23) are superimposed (root mean square deviation = 0.9 Å for structurally conserved BIR domain heavy atoms). The peptide ligand bound to ML-IAP is shown as yellow sticks. There is no overlap of IAP-like peptide- and Borealin-binding sites on the surface of the Survivin monomer. All structure figures were produced using the program PyMOL (24).

The above figures are reprinted by permission from the ASBMB: J Biol Chem (2007, 282, 35018-35023) copyright 2007.