FIGURE 4:

The chymotrypsin-like serine protease domain in SltB. (A) Diagram focusing on the relevant region, showing the conservation of putative key HDS (His/Asp/Ser) triad residues, constituting the catalytic domain of a serine protease. Logos indicate sequence conservation in residues putatively constituting the catalytic triad and flanking regions. (B) Two views of the model generated by the Phyre2 server of the SltB fragment between coordinates 900 and 1192. Left, rainbow representation (see text). Indicated in purple are the positions of Gly-1156 and Phe-1126, in which mutations suppressing null vps alleles have been selected. See Discussion and Mellado et al. (2015) for Gly1156 and Results for the sltB57 mutation deleting F1126. Right, focused (partial) view of the possible catalytic site of the SltB protease. S1142, D1037, and several His residues that might form part of the triad are indicated. Mutations affecting S1142 and H1033 are described in Results. (C) Phenotype analysis of mutant strains expressing chimeras carrying alanine substitutions for either S1142 or H1033. (D) Immunodetection of WT and mutant tagged forms of SltB and the effects of mutations S1142A and H1033A on the proteolysis of SltA-HA₃.