Insights into retinal disease and non-tubulin glutamylation from a RPGR-TTLL5 complex structure

Mutations in retinitis pigmentosa GTPase regulator (RPGR) cause photoreceptor degeneration, vision loss, and eventual blindness. RPGR function requires glutamylation by tubulin tyrosine ligase-like 5 (TTLL5) whose mutation is also linked to severe forms of retinal degeneration. How TTLL5 targets RPGR and how mutations in either protein cause disease are unknown. Here we report the 2.8-Å X-ray crystal structure of the coactivator interacting domain (CID) of human TTLL5 in complex with the RPGR C terminus, both required for glutamylation. The RPGR C terminus forms a helix that intercalates through aromatic interactions into the CID helical bundle of novel fold. Interfacial residues are mutated in retinitis pigmentosa, as well as macular degeneration of unknown etiology. Key mutations at this interface abolish RPGR-TTLL5 interaction in vitro and RPGR glutamylation in mouse photoreceptors. Our work reveals mechanisms of non-tubulin substrate recognition by TTLL glutamylases, increasingly recognized as broad regulators of the proteome, and sheds light on mechanisms of disease associated with TTLL5 and RPGR mutations.