Crystal structure of Bacillus subtilis SPP1 phage gp23.1, a putative chaperone

Abstract

SPP1 is a siphophage infecting the gram-positive bacterium Bacillus subtilis. The SPP1 tail electron microscopy (EM) reconstruction revealed that it is mainly constituted by conserved structural proteins such as the major tail proteins (gp17.1), the tape measure protein (gp18), the Distal tail protein (Dit, gp19.1), and the Tail associated lysin (gp21). A group of five small genes (22-24.1) follows in the genome but it remains to be elucidated whether their protein products belong or not to the tail. Noteworthy, an unassigned EM density accounting for ~245 kDa is present at the distal end of the SPP1 tail-tip. We report here the gp23.1 crystal structure at 1.6 A resolution, a protein that lacks sequence identity to any known protein. We found that gp23.1 forms a hexamer both in the crystal lattice and in solution as revealed by light scattering measurements. The gp23.1 hexamer does not fit well in the unassigned SPP1 tail-tip EM density and we hypothesize that this protein might act as a chaperone.

Figure 1

Overall phage SPP1 gp23.1 structure. A: Stereo ribbon representation of the gp23.1 monomer. The protein is colored in rainbow representation from the N-terminus (blue) toward the C-terminus (red). B: Ribbon representation of the gp23.1 hexamer. Each monomer is colored individually. C: Surface representation of the gp23.1 hexamer according to the same colour scheme as in B. D: Crystal packing in the hexagonal space group. A tube is formed through stacking of hexamers with the same orientation. E: Gp23.1 electrostatic potential. The hexamer is rotated 180° relative to C. F: Clipped view of gp23.1 exhibiting the asymmetrical negative charge distribution of the central channel. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]