The S24, S26 LexA/signal peptidase superfamily contains LexA-related and type I signal peptidase families. The S24 LexA protein domains include: the lambda repressor CI/C2 family and related bacterial prophage repressor proteins; LexA (EC 3.4.21.88), the repressor of genes in the cellular SOS response to DNA damage; MucA and the related UmuD proteins, which are lesion-bypass DNA polymerases, induced in response to mitogenic DNA damage; RulA, a component of the rulAB locus that confers resistance to UV, and RuvA, which is a component of the RuvABC resolvasome that catalyzes the resolution of Holliday junctions that arise during genetic recombination and DNA repair. The S26 type I signal peptidase (SPase) family also includes mitochondrial inner membrane protease (IMP)-like members. SPases are essential membrane-bound proteases which function to cleave away the amino-terminal signal peptide from the translocated pre-protein, thus playing a crucial role in the transport of proteins across membranes in all living organisms. All members in this superfamily are unique serine proteases that carry out catalysis using a serine/lysine dyad instead of the prototypical serine/histidine/aspartic acid triad found in most serine proteases.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1371208112  31 PILLSYVTTDSMEPALSPGDgFVAVPSAIAGDVGPGDVVVYEAEqiEGGGLTTHRVVE----ETERGYVTRGDANPFTD- 105
Cdd:TIGR02228  30 PDPVVVVLSGSMEPTFNTGD-LILVTGADPNDIQVGDVITYKSP--GFNTPVTHRVIEinnsGGELGFITKGDNNPAPDg 106

                          90       100       110       120
                  ....*....|....*....|....*....|....*....|...
gi 1371208112 106 ------------QGSGEPPVQDATVVAVAWQPTGSLLAIPGLG 136
Cdd:TIGR02228 107 epvpsenvigkyLGFTIPFAGYVLVFAPQAIGAAALLIIPGIG 149

signal peptidase I, archaeal type; This model represents signal peptidase I from most archaea, a subunit of the eukaryotic endoplasmic reticulum signal peptidase I complex, and an apparent signal peptidase I from a small number of bacteria. It is related to but does not overlap in hits with TIGR02227, the bacterial and mitochondrial signal peptidase I.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1371208112  31 PILLSYVTTDSMEPALSPGDgFVAVPSAIAGDVGPGDVVVYEAEqiEGGGLTTHRVVE----ETERGYVTRGDANPFTD- 105
Cdd:TIGR02228  30 PDPVVVVLSGSMEPTFNTGD-LILVTGADPNDIQVGDVITYKSP--GFNTPVTHRVIEinnsGGELGFITKGDNNPAPDg 106

                          90       100       110       120
                  ....*....|....*....|....*....|....*....|...
gi 1371208112 106 ------------QGSGEPPVQDATVVAVAWQPTGSLLAIPGLG 136
Cdd:TIGR02228 107 epvpsenvigkyLGFTIPFAGYVLVFAPQAIGAAALLIIPGIG 149

signal peptidase I, archaeal type; This model represents signal peptidase I from most archaea, a subunit of the eukaryotic endoplasmic reticulum signal peptidase I complex, and an apparent signal peptidase I from a small number of bacteria. It is related to but does not overlap in hits with TIGR02227, the bacterial and mitochondrial signal peptidase I.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1371208112  31 PILLSYVTTDSMEPALSPGDgFVAVPSAIAGDVGPGDVVVYEAEqiEGGGLTTHRVVE----ETERGYVTRGDANPFTD- 105
Cdd:TIGR02228  30 PDPVVVVLSGSMEPTFNTGD-LILVTGADPNDIQVGDVITYKSP--GFNTPVTHRVIEinnsGGELGFITKGDNNPAPDg 106

                          90       100       110       120
                  ....*....|....*....|....*....|....*....|...
gi 1371208112 106 ------------QGSGEPPVQDATVVAVAWQPTGSLLAIPGLG 136
Cdd:TIGR02228 107 epvpsenvigkyLGFTIPFAGYVLVFAPQAIGAAALLIIPGIG 149

The S26 Type I signal peptidase (SPase; LepB; leader peptidase B; leader peptidase I; EC 3.4.21.89) family members are essential membrane-bound serine proteases that function to cleave the amino-terminal signal peptide extension from proteins that are translocated across biological membranes. The bacterial signal peptidase I, which is the most intensively studied, has two N-terminal transmembrane segments inserted in the plasma membrane and a hydrophilic, C-terminal catalytic region that is located in the periplasmic space. Although the bacterial signal peptidase I is monomeric, signal peptidases of eukaryotic cells commonly function as oligomeric complexes containing two divergent copies of the catalytic monomer. These are the IMP1 and IMP2 signal peptidases of the mitochondrial inner membrane that remove leader peptides from nuclear- and mitochondrial-encoded proteins. Also, two components of the endoplasmic reticulum signal peptidase in mammals (18-kDa and 21-kDa) belong to this family and they process many proteins that enter the ER for retention or for export to the Golgi apparatus, secretory vesicles, plasma membranes or vacuole. An atypical member of the S26 SPase type I family is the TraF peptidase which has the remarkable activity of producing a cyclic protein of the Pseudomonas pilin system. The type I signal peptidases are unique serine proteases that utilize a serine/lysine catalytic dyad mechanism in place of the classical serine/histidine/aspartic acid catalytic triad mechanism.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1371208112  34 LSYVTTDSMEPALSPGDGFVAVP-SAIAGDVGPGDVVVYEAEQiEGGGLTTHRVVeeterGYVTRGDA--NPFTDQGSGE 110
Cdd:cd06530     2 PVVVPGGSMEPTLQPGDLVLVNKlSYGFREPKRGDVVVFKSPG-DPGKPIIKRVI-----GYFVLGDNrnNSLDSRYWGP 75

                          90
                  ....*....|..
gi 1371208112 111 PPVQDatVVAVA 122
Cdd:cd06530    76 VPEDD--IVGKV 85