9.A.31 The Putative SdpC Peptide Antibiotic-like Killing Factor Exporter, SdpAB (SdpAB) Family
Ellermeier et al. (2006) have identified an exporter, SdpAB, that expels a 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) show very few homologues in the 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, its only paralogue. 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.
The presumed transport reaction is:
SdpC (in) → SdpC (out)
SdpB homologue of Streptomyces coelicolor
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, Q6MLM8 and Q6MQM7.
SdpB1/2 of Bdellovibrio bacteriovorus
SdpB1, 287 aas; 6 TMSs
SdpB2, 308 aas; 6 TM