Prominin-1, a five-transmembrane glycoprotein, is selectively associated with membrane protrusions such as microvilli, filopodia and lamellipodia and is concentrated at the base of the rod outer segments (ROS) in the mouse retina. Mutations of prominin-1 cause recessive retinal degeneration in human. Little is known, however, about the expression and precise localization of this protein in photoreceptors. Here I show evidence from phylogenetic studies that prominin genes underwent duplication and deletion during evolution. I discovered a total of three prominin homologues (xProminin-1, 2 and 3) in X. laevis , an animal model used in my studies. Semi-quantitative RT-PCR shows that mRNAs of xProminin-1, 2 and 3 are co-expressed with each other in retina, brain, testis and kidney but differ greatly in their expression levels in these tissues. The mRNAs of all three prominin homologues are alternatively spliced. Both anti-xProminin-1 N and C-terminal antibodies label the base of rod outer segments as a thin band and also faintly label the entire ROS in a dispersed distribution. The intensity of immunolabeling at the base of ROS shows variation during the day, a phenomenon possibly regulated by entrainment of light and dark as well as other yet unknown factors. Both antibodies also label the entire outer segments of cones asymmetrically on one side without obvious variation in labeling intensity during the day. The labeling of prominin-1 on the cone outer segments is restricted to the open rims of lamellae, in contrast with peripherin-2/rds, which labels the opposite, closed rims of lamellae that are adjacent to the ciliary axoneme. The association of xProminin-1 with open rims of disk lamellae is further supported by immuno electron microscopy study and by xProminin-1-hrGFP expression in both rod and cone photoreceptors of transgenic tadpoles. Similar localization of prominin-1 in murine rods and cones is also observed. These findings indicate that xProminin-1 may participate in the process of disk morphogenesis in photoreceptors by preventing fusion of membrane.
