The family of bacterial Anabaena sensory rhodopsin transducers are likely to bind sugars or related metabolites. The entire protein is comprised of a single globular domain with an eight-stranded beta-sandwich fold. There are a few characteristics which define this beta-sandwich fold as being distinct from other so-named folds, and these are: 1) a well conserved tryptophan, usually following a polar residue, present at the start of the first strand; this tryptophan appears to be central to a hydrophobic interaction required to hold the two beta-sheets of the sandwich together, and 2) a nearly absolutely conserved asparagine located at the end of the second beta-strand, that hydrogen bonds with the backbone carbonyls of the residues 2 and 4 positions downstream from it, thereby stabilising the characteristic tight turn between strands 2 and 3 of the structure.

1: Demonstration of a sensory rhodopsin in eubacteria. Jung KH, Trivedi VD, Spudich JL; Mol Microbiol. 2003;47:1513-1522. PMID:12622809

2: The Anabaena sensory rhodopsin transducer defines a novel superfamily of prokaryotic small-molecule binding domains. De Souza RF, Iyer LM, Aravind L; Biol Direct. 2009; [Epub ahead of print] PMID:19682383