2.A.122 The LrgB/CidB Holin-like Glycolate/Glycerate Transporter (LrgB/CidB/GGT) Family
CidA and LrgA are a putative holin/antiholin pair in Staphylococcus aureus. LrgB is LrgA-associated (lrgA and lrgB are translationally coupled) while CidB is CidA-associated (cidA and cidB are similarly translationally coupled). Both have been reported to play a role in cell wall hydrolysis, possibly by regulating murein hydrolase export. The cidABC operon is controlled by CidR, encoded adjacent to the cidABC operon, and the lrgABC operon is controlled by LrgR, encoded upstream of the lrgABC operon. Cell lysis results in response to acetic acid accumulation in the medium (Yang et al. 2005).
A longer homologue of the bacterial LrgB has been shown to be the plastidic glycolate glycerate transporter, PLGG1, of 512 aas and 12 TMSs in Arabidopsis thaliana (Pick et al. 2013). The last 5 TMSs are homologous to the 5 TMSs in several CidB and LrgB proteins of bacteria (see proteins 2.A.122.1.1 and 1.2). This plant protein may represent a fusion of the bacterial LrgA and LrgB proteins (Wang and Bayles 2013). Because of this function, it appears that at least this plant protein of 12 TMSs must be a secondary carrier.