Table 1.

Key actors in cysteine-mediated envelope homeostasis in E. coli

Enzyme	Function
Envelope oxidoreductases
Disulfide bond formation
DsbA	Catalyzes disulfide bond formation in the periplasm
DsbB	Inner membrane protein that recycles DsbA
Disulfide bond isomerization
DsbC	Catalyzes disulfide bond isomerization in the periplasm
DsbD	Inner membrane protein that recycles DsbC
Cysteine protection
DsbG	Rescues periplasmic single cysteine residues from oxidative damage
DsbD	Inner membrane protein that recycles DsbG
Essential envelope assembly factors with structural disulfide bonds
LptD	Outer membrane protein that inserts lipopolysaccharide molecules into the membrane; two nonconsecutive disulfides; formation of at least one disulfide is essential
BamA	Outer membrane protein that inserts β-barrel proteins into the outer membrane; one nonessential disulfide bond
FtsN	Inner membrane protein with a large periplasmic domain regulating peptidoglycan synthesis; one essential disulfide bond in the periplasmic domain.
Disulfide-containing stress sensors monitoring envelope integrity
RcsF	Outer membrane lipoprotein monitoring the integrity of the peptidoglycan and of the outer membrane; two nonconsecutive disulfides required for folding; induces the Rcs phosphorelay pathway under stress
NlpE	Outer membrane lipoprotein monitoring lipoprotein trafficking to the outer membrane; two consecutive disulfides required for folding; induces the Cpx system when lipoprotein transport is perturbed
Envelope assembly enzymes with a catalytic cysteine residue
LdtA, LdtB, and LdtC	l,d-Transpeptidases catalyzing the attachment of the Braun lipoprotein Lpp to the peptidoglycan
LdtD and LdtE	l,d-Transpeptidases catalyzing the formation of 3-3 cross-links between two meso-diaminopimelic acid residues of adjacent stem peptides