In vertebrate photoreceptors, opsins are highly concentrated in a morphologically distinct ciliary compartment known as the outer segment (OS). Opsin is synthesized in the cell body and transported to the OS at a remarkable rate of 100-1000 molecules per second. Opsin transport defects contribute to photoreceptor loss and blindness in human ciliopathies. Previous studies revealed that the opsin C-terminal tail, of 44 amino acids, is sufficient to mediate OS targeting in Xenopus photoreceptors. Here we show that although the Xenopus C-terminus retains this function in zebrafish, the homologous zebrafish sequence is not sufficient to target opsin to the OS. This functional difference is largely caused by a change of a single amino acid present in Xenopus, but not in other vertebrates examined. Furthermore, we find that sequences in the 3rd intracellular cytoplasmic loop (IC3) and adjacent regions of transmembrane helixes 6 and 7 are also necessary for opsin transport in zebrafish. Combined with the cytoplasmic tail, these sequences are sufficient to target opsin to the ciliary compartment.
Identification of additional outer segment targeting signals in zebrafish rod opsin.
