Architecture of staphylococcal HtrA proteins and design of soluble proteins. Three HtrA family members are shown: from top to bottom, E. coli DegS, S. aureus HtrA1 and HtrA2 proteins (strain COL). They display the typical domain organisation of the family: transmembrane, catalytic and PDZ domains are shown as hatched, dark grey and light grey boxes respectively, with their boundaries (residue position) indicated below. The catalytic Serine residue (S, in bold) is shown. The recombinant DegS form whose structure has been solved after N-terminal transmembrane domain deletion  is named DegSΔTM. A similar deletion strategy was applied to HtrA1 and HtrA2 proteins leading to N-terminally truncated proteins named HtrA1ΔTM and HtrA2ΔTM. All truncated protein forms are shown as lines with the position of their first and last residues in the corresponding WT sequence indicated.