Fig. 1: Structure of Ssp.

a Linear schematic of Ssp’s primary sequence encompassing an N-terminal signal peptide (SP) and a C-terminal translocator domain flanking the central passenger. Active site residues in the subtilase domain are shown by red line, self-cleavage sites shown by green arrows. b Crystal structure showing overall architecture of Ssp passenger in cartoon representation. Subtilase (protease) domain is depicted in blue with active site residues shown as red spheres. The β-helical stalk domain is represented in yellow, with protruding β-helix Loop 1 and Loop 2, coloured in rust and green, respectively. The passenger-associated-transport-repeat (PATR) is displayed in pink and the RGD motif shown as pink sticks, the α-helical loop in orange and the β-hairpin cap at the base of the passenger are coloured in dark blue. Calcium ions are shown as pink spheres with those bound to the protease domain labelled using Dohnalek et al. nomenclature⁵⁷. c Protease domain of Ssp in cartoon representation showing the unique active site protrusions (E1–E3). Namely, short β-hairpin extension (E1, pink) long β-hairpin extension (E3, green), and extended loop extension with connected α-helix (E2, orange). Active site is displayed as red sticks. d Ssp protease domain with central β-sheet and α-helices shown in hot pink and jade, respectively. Disulfide bond is shown as yellow sticks.