Tag:ePDZb1 is an engineered PDZ domain that binds the DKAVDTWV C-terminal peptide epitope of Tag:LOVpep with high affinity. Tag:ePDZb1 can be used in combination with Tag:LOVpep 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. In the dark, binding of Tag:ePDZb1 and Tag:LOVpep is inhibited due to caging of the DKAVDTWV epitope in Tag:LOVpep by the Jα helix binding to the LOV2 core. Upon blue light irradiation, unfolding of the Jα helix exposes the C-terminal peptide epitope in Tag:LOVpep, allowing it to bind Tag:ePDZb1 and form a heterodimer (PMID:22388287).