The Late Endosomal Pathway Regulates the Ciliary Targeting of Tetraspanin Protein Peripherin 2.

Peripherin 2 (PRPH2) is a tetraspanin protein concentrated in the light-sensing cilium (called the outer segment) of the vertebrate photoreceptor. The mechanism underlying the ciliary targeting of PRPH2 and the etiology of cone dystrophy caused by PRPH2 mutations remain elusive. Here we show that the late endosome (LE) is the main waystation that critically sorts newly synthesized PRPH2 to the cilium. PRPH2 is expressed in the luminal membrane of the LE. We delineate multiple C-terminal motifs of PRPH2 that distinctively regulate its LE and ciliary targeting through ubiquitination and binding to ESCRT (Endosomal Sorting Complexes Required for Transport) component Hrs. Using the newly developed TetOn-inducible system in transfected male and female mouse cones in vivo, we show that the entry of nascent PRPH2 into the cone outer segment can be blocked by either cone dystrophy-causing C-terminal mutations of PRPH2, or by short-term perturbation of the LE or recycling endosomal traffic. These findings open new avenues of research to explore the biological role of the LE in the biosynthetic pathway and the etiology of cone dystrophy caused by PRPH2 mutations and/or malfunctions of the LE.