STRUCTURE NOTE Crystal Structure of a Truncated Version of the Phage Protein gpD

Introduction. Bacteriophage contains a linear doublestranded DNA genome enclosed within an icosahedral capsid of triangulation number T 7, and a flexible, non-contractile, tail. It has been adapted for phage display, providing a valuable alternative to filamentous phages. As a lytic phage, the phage coat is directly assembled in the cytoplasm, and does not pass through the secretion machinery and the oxidizing periplasm, thereby providing a display system for proteins not suitable for secretion. Both the head protein, gpD, and the tail protein, gpV, have been used as fusion partners in applications of phage display. To further develop the technology of phage display, the structures of the proteins utilized in this technique are of considerable interest. The high-resolution crystal structure of gpD, the capsid-stabilizing protein of bacteriophage , which sits as protrusions at trigonal sites, has been solved previously. It provides a structural basis for the understanding of how gpD could play the role of a display platform. Molecules of gpD were found to be arranged in noncrystallographic trimers with substantial inter-subunit interfaces. Their C termini are well ordered and located on one side of the trimer, relatively far from its threefold axis. By contrast, the N termini are disordered up to Ser15, which is close to the threefold axis and on the same side as the C termini. Even though these 14 N-terminal residues are disordered, they appear to have an important function, since an N-terminal truncated version of gpD (gpD N1) does not complement a phage mutant devoid of gpD. Both gpD and gpD N1 are monomeric in solution, and gpD trimerizes on the phage and upon crystallization. It was, therefore, of interest to characterize gpD N1 more closely. While gpD N1 contains all visible atoms of the gpD trimer, it was conceivable that the N-terminal stretch might play an important role in trimer formation, which might occur concomitant with binding to the phage procoat. Furthermore, Pro17 and His19 form a characteristic ring in the trimer, and this might be compromised when the first 14 residues are absent. Finally, we considered that if crystals of gpD N1 isomorphous to those of the full-length protein could be obtained, comparison of the low-resolution diffraction of both forms might conceivably elucidate the location of the N terminus of the molecule, even if disordered, by applying an approach similar to the X-ray contrast variation used to outline the protein molecules. We thus solved the structure of gpD N1 at the resolution of 1.85 Å. Despite having crystallized in a larger cell with two trimers in the asymmetric unit, the structure of this truncated version of gpD is virtually identical to the structure of the unmodified protein.