Biochemical and structural characterization of a tail-spike protein with depolymerase activity identified in a marine podovirus

Marine phages are, through the infection of their bacterial hosts, key regulators of microbiome and carbon fluxes in the ocean. Despite their important role, the specific molecular mechanisms that underlie infection are so far understudied. Previously, the podovirus Cobetia marina virus 1 (Carin-1), which infects the marine γ-proteobacterium C. marina, was shown to display exopolysaccharide depolymerase activity. This activity is likely to mediate degradation of the host capsule to facilitate access to the bacterial membrane receptor, but no corresponding gene could be annotated in the genome of Carin-1 by comparative genomics. Biochemical characterization enabled assignment of this activity to Dpo31, a protein sharing less than 10% sequence identity with any characterized protein. Here, we report the structural domain organization and biochemical characterization of Dpo31, revealing an overall structure that is analogous to podovirus tail-spike proteins, allowing us to locate the depolymerase activity to the D3 domain and to identify original structural features that explain the absence of detectable similarity at the primary-sequence level.