9.A.31 The Putative SdpC Peptide Antibiotic-like Killing Factor Exporter, SdpAB (SdpAB) Family

Ellermeier et al. (2006) have identified a putative exporter, SdpAB, that was thought to expels the toxic peptide, SdpC from the cell cytoplasm of Bacillus subtilis. Extracellular SdpC induces synthesis of an immunity protein, SdpI (TC #9.A.32) that protects cells from being killed. SdpI also functions in signal transduction by binding and sequestering the SdpR autorepressor at the membrane when SdpC is bound. SdpA (YvaW) and SdpB (YvaX) have few homologues in the NCBI database, and these are primarily found in Bacillus species. They show no significant sequence similarity with any protein of known function. SdpA is 158 aas long and is hydrophilic with a single N-terminal mildly hydrophilic stretch of 18 residues with no charged residues in it. This region is followed by a strongly hydrophilic region (PQNPLFKKNFLQQ), resembling in this respect YitP (CAB12947) of B. subtilis. SdpB has 6 putative TMSs in a 2 + 2 + 2 arrangement and is 323 aas long. It shows similarity throughout most of its length with the horizontally transferred transmembrane (HTTM) domain in CDD (CDD25353) which has 4 TMSs and is found in proteins from the 3 domains of life. This domain is found in the N-termini of vitamin K-dependent γ-carboxylases. However, Pérez Morales et al. 2013 concluded that SdpAB are not required for secretion, translation, or stability of SdpC, and that they may participate in a posttranslation step in the production of SDP. The mature form of the SDP toxin contains a disulfide bond which increases the activity of SDP, it is not essential for it; the disulfide bond forms independently of SdpAB. Thus, SDP production is a multistep process in which SdpAB are required for SDP production, likely by controlling, directly or indirectly, cleavage of SDP from the pro-SdpC precursor (Pérez Morales et al. 2013).

The putative transport reaction is:

SdpC (in) → SdpC (out)

but the reaction catalyzed by SdpAB may be:

ProSdpC (out) → proteolticallly processed and mature SdpC (SDP)


 

References:

Ellermeier, C.D., E.C. Hobbs, J.E. Gonzalez-Pastor, and R. Losick. (2006). A three-protein signaling pathway governing immunity to a bacterial cannibalism toxin. Cell 124: 549-559.

Kobayashi, K. and Y. Ikemoto. (2019). Biofilm-associated toxin and extracellular protease cooperatively suppress competitors in Bacillus subtilis biofilms. PLoS Genet 15: e1008232.

PĂ©rez Morales, T.G., T.D. Ho, W.T. Liu, P.C. Dorrestein, and C.D. Ellermeier. (2013). Production of the cannibalism toxin SDP is a multistep process that requires SdpA and SdpB. J. Bacteriol. 195: 3244-3251.

Examples:

TC#NameOrganismal TypeExample
9.A.31.1.1

The putative peptide antibiotic-like killing factor (SdpC) exporter, SdpAB. However, Pérez Morales et al. 2013 concluded that SdpAB are not required for secretion, translation, or stability of SdpC, and that they may participate in a posttranslation step in the production of SDP. The mature form of the SDP toxin contains a disulfide bond. Their data indicate that while the disulfide bond does increase activity of SDP, it is not essential for SDP activity, and that the disulfide bond is formed independently of SdpAB. Thus, SDP production is a multistep process in which SdpAB are required for SDP production, likely by controlling, directly or indirectly, cleavage of SDP from the pro-SdpC precursor (Pérez Morales et al. 2013).

Bacteria

SdpAB of Bacillus subtilis
SdpA (YvaW) (CAB15380)
SdpB (YvaX) (CAB15381)

 
9.A.31.1.2

YvaB homologue of 436 aas and 10 or 11 TMSs.  It is encoded by a gene that is adjacent to one that probably codes for an outer membrane porin with TC# 1.B.89.1.21.

Actinobacteria

SdpB homologue of Streptomyces coelicolor

 
9.A.31.1.3

Two component uncharacterized homologues of SdpB (YvaX) of 287 aas and 308 aas, respectively, both with 6 TMSs.  Their genes are adjacent to each other but transcribed devergently.  According to GO, they may be γ-glutamyl carboxylases or peptidyl glutamate carboxylases.  Two other such homologues are encoded in the same genome (see 9.A.31.1.5).

 

SdpB1/2 of Bdellovibrio bacteriovorus
SdpB1, 287 aas; 6 TMSs
SdpB2, 308 aas; 6 TM

 
9.A.31.1.4

YitOP, a probable two component YIT killer factor (TC# 9.B.139.1.6) export system (Kobayashi and Ikemoto 2019).

YitOP of Bacillus subtilis
YitO, 309 aas and 6 TMSs, O06750
YitP, 178 aas and 1 TMS, N-terminal, O06751

 
9.A.31.1.5

Putative transporter consisting of two HTTM domain-containing proteins of 299 aas and 6 TMSs as well as 291 and possibly 7 or 8 TMSs.  These two proteins are homologous to each other and the two proteins with TC# 9.A.31.1.2.

Putative two component transporter of Bdellovibrio bacteriovorus

 
Examples:

TC#NameOrganismal TypeExample