The illustration shows a detailed model of how T T L L 5 and R P G R mutations affect different cells in the retina and male reproductive system. On the left, it depicts the eye and testis, zooming into the retina and healthy sperm. In the retina, healthy cones are shown with normal opsin transport through the connecting cilium containing glutamylated R P G R to the outer segment. In contrast, mutated cones display impaired opsin transport due to unglutamylated R P G R. In healthy sperm, glutamylated axonemal microtubules ensure normal axoneme structure. Mutated sperm show hypoglutamylated axonemal microtubules, missing axonemal doublet 4, leading to compromised morphology and motility. On the right, the illustration explains how different mutations in R P G R and T T L L 5 impact these processes. Mutations in the R P G R mini B D region and point mutations in the T T L L 5 C I D specifically affect R P G R glutamylation in photoreceptors but not tubulin glutamylation in sperm axoneme. Mutations in the T T L L 5 catalytic domain or M T B D cause loss of both R P G R and tubulin glutamylation. Additionally, mutations in the T T L L 5 R I D may cause retinal degeneration by impacting protein stability, localization, or recognition of non-T T L L 5 and non-R P G R substrates. The illustration includes labels for various domains and mutations, highlighting their specific impacts on cellular functions.
Proposed model for the division of substrate recognition between the TTLL5 core and CID leading to distinct disease manifestations. Schematic showing how RPGR and TTLL5 mutations impact distinct specialized cells in retina and male reproductive system (left). In healthy photoreceptors, opsin (red dots) is trafficked (red arrow) through the connecting cilium containing glutamylated RPGR to the outer segment. In RPGR/TTLL5 mutations, opsin trafficking is disrupted, leading to outer segment degeneration (middle, top). In healthy sperm, TTLL5 glutamylates tubulin in axonemes (axonemal doublets in dark and light green). When TTLL5 recognition of tubulin is disrupted, axonemal glutamylation is insufficient, and sperm morphology and motility are compromised (middle, bottom). Right: at the protein level, our results suggest that mutations in RPGR miniBD region and point mutations in TTLL5 CID specifically impact RPGR glutamylation in photoreceptors but not tubulin glutamylation in sperm axoneme. Mutations in TTLL5 catalytic domain or MTBD cause loss of both RPGR and tubulin glutamylation. Additionally, mutations in TTLL5 RID may cause retinal degeneration by impacting protein stability, localization, or recognition of non-TTLL5 and non-RPGR substrates. TTLL5 and RPGR domains colored according to Fig. 1 A. Disease-associated RPGRORF15 and TTLL5 mutations reported previously (Bedoni et al., 2016) are indicated in black on the protein schematic. Additional mutations from the ClinVar database that we predict cause retina-specific disease are shown in gray.