8.A.59 The SLC and TCST-Associated Component (STAC-A) Family
Transmembrane receptors, integral components of sensory pathways in prokaryotes, share a common dimeric architecture, consisting in its basic form of an N-terminal extracellular sensor, two transmembrane helices, and an intracellular effector. An archaeal receptor, Af1503 (here designated STAC-A) from Archaeoglobus fulgidus has the two usual TMSs, but is C-terminally shortened, lacking a recognizable effector module. Instead, a HAMP domain forms the sole extension for signal transduction in the cytosol (Korycinski et al. 2015). Members of this family are found both as stand-alone proteins and as domains within extant receptors. In general, the latter appear as connectors between members of the Solute:Sodium Symporter Family (TC#2.A.21; SLC5) transmembrane domains and two-component signal transduction domains. This is seen, for example, in the histidine kinase CbrA, which is a global regulator of metabolism, virulence, and antibiotic resistance in Pseudomonads. This domain family mediates signal transduction in systems regulating transport processes (Korycinski et al. 2015).