Crystal structure of the C-terminal region of human p97/DAP5.

Translation initiation in eukaryotes is mediated by a large group of eukaryotic translation initiation factors (eIFs) that coordinate the recruitment of 50-capped mRNAs and ribosomal subunits. Human eIF4GI is a scaffold protein which interacts with other eIFs in various ways: in a large complex, elF4F, with eIF4A and eIF4E; the interaction between eIF3 and eIF4GI places the 40S ribosome next to the 50 end of the mRNA1; other binding partners include eIF2, Mnk1 and PABP. eIF4GI consists of three regions (determined by viral protease cleavage patterns) of approximately 500 amino acids each: the Tyr-X4Leu-F binding site for eIF4E (found in the N-terminal region of the protein) and the middle region of eIF4GI (eIF4GI/M) are essential for cap-dependent translation2; the C-terminal region (eIF4GI/C) is present only in higher eukaryotes and is known to modulate protein synthesis.3 p97 [also known as death-associated protein 5 (DAP5)] shows high homology to the C-terminal two-thirds of eIF4GI and is a highly conserved, ubiquitously expressed eukaryotic protein.4,5 Because it lacks the N-terminal region of eIF4GI, p97 cannot interact with eIF4E and PABP, but can interact with eIF4A and eIF36,7 and is therefore thought to act as a dominant negative inhibitor of translation initiation under normal cellular conditions by mimicking eIF4GI.4 Other putative functions for the full length protein include a paradoxical role as a translational activator in unstressed cells.8 Furthermore, a p86 fragment (liberated by caspase cleavage) is thought to mediate cap-independent translation in apoptotic cells.9 Liberman et al.10 reported the crystal structure of the C-terminal region (residues 730– 897) of DAP5/p97, which provided insightful structural information of its processing by caspase cleavage. In this study, the crystal structure of the C-terminal region (residues 540–897) of human p97 (p97/C), which has low sequence identity (27%) with corresponding Cterminal region of human eIF4GI, was determined at 2.0 Å resolution. The structure displays two atypical HEAT (Huntingtin, EF3, PP2A, and TOR1) domains, p97/C1 and p97/ C2. They are linked by a short 310-helix, which makes the whole p97/C have a rod-like structure. Unlike the eIF4A binding surface of MA3 domain of human eIF4GI which has a highly negatively charged surface, the corresponding area of the MA3 domain of human p97 (p97-MA3) displays a continuously positively charged patch, which could prevent p97-MA3 from binding to the eIF4A.