A motif foramino acid recognition by proteins or domains of the periplasmic binding protein-like I superfamily has been identified. An initial pattern of 5 residues was based on a multiple sequence alignment of selected proteins of that fold family and on common structural features observed in the crystal structure of some members of the family [leucine isoleucine valine binding protein (LIVBP), leucine binding protein (LBP), and metabotropic glutamate receptor type 1 (mGlu1R) amino terminal domain)]. This pattern was used against the PIR-NREF sequence database and further refined to retrieve all sequences of proteins that belong to the family and eliminate those that do not belong to it. A motif of 8 residues was finally selected to build up the general signature. A total of 232 sequences were retrieved. They were found to belong to only three families of proteins: bacterial periplasmic binding proteins (PBP, 71 sequences), family 3 (or C) of G-protein coupled receptor (GPCR) (146 sequences), and plant putative ionotropic glutamate receptors (iGluR, 15 sequences). PBPs are known to adopt a bilobate structure also named Venus flytrap domain, or LIVBP domain in the present case. Family 3/C GPCRs are also known to hold such a domain. However, for plant iGluRs, it was previously detected by classical similarity searches but not specifically described. Thus plant iGluRs carry two Venus flytrap domains, one that binds glutamate and an additional one that would be a modulatory LIVBP domain. In some cases, the modulator binding to that domain would be an amino acid.
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