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)