The crystal structure of the non-liganded 14-3-3σ protein: insights into determinants of isoform specific ligand binding and dimerization

摘要

Seven different, but highly conserved 14-3-3 proteins are involved in diverse signaling pathways in human cells. It isunclear how the 14-3-3σ isoform, a transcriptional target of p53, exerts its inhibitory effect on the cell cycle in thepresence of other 14-3-3 isoforms, which are constitutively expressed at high levels. In order to identify structuraldifferences between the 14-3-3 isoforms, we solved the crystal structure of the human 14-3-3σ protein at a resolutionof 2.8 ? and compared it to the known structures of 14-3-3ζ and 14-3-3τ. The global architecture of the 14-3-3σ foldis similar to the previously determined structures of 14-3-3ζ and 14-3-3τ: two 14-3-3σ molecules form a cup-shapeddimer. Significant differences between these 14-3-3 isoforms were detected adjacent to the amphipathic groove, whichmediates the binding to phosphorylated consensus motifs in 14-3-3-ligands. Another specificity determining region islocalized between amino-acids 203 to 215. These differences presumably select for the interaction with specific ligands,which may explain the different biological functions of the respective 14-3-3 isoforms. Furthermore, the two 14-3-3σmolecules forming a dimer differ by the spatial position of the ninth helix, which is shifted to the inside of the ligandinteraction surface, thus indicating adaptability of this part of the molecule. In addition, 5 non-conserved residues arelocated at the interface between two 14-3-3σ proteins forming a dimer and represent candidate determinants of homo-and hetero-dimerization specificity. The structural differences among the 14-3-3 isoforms described here presumablycontribute to isoform-specific interactions and functions.