Tag:SSPB(R73Q) corresponds to the adaptor protein encoded by the Escherichia coli sspB gene (ECOGENE:EG10978), mutated to carry a R73Q amino acid substitution. Tag:SSPB(R73Q) is bound specifically by Tag:iLID in a light-dependent manner. In the dark, binding of Tag:iLID to Tag:SSPB(R73Q) is sterically inhibited, while upon blue light irradiation, a conformational change in Tag:iLID allows it to bind to Tag:SSPB(R73Q). This property can be used to bring together two tagged proteins of interest in vivo in a light-dependent manner, for example to reconstitute a functional protein by bringing together complementary split system components, or to target a protein of interest to a particular subcellular location (PMID:25535392).