Molecular bases of disease expression of syndromic visual dystrophies by RPGR‐dependent and differential subcellular sorting and processing of RPGRIP1

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RPGR and RPGRIP1 play direct roles in syndromic retinal dystrophies by elusive mechanisms. RPGR interacts with the RPGRIP1 via its RHD domain, which is homologous to RCC1, a nucleotide‐exchange factor for RAN GTPase. Molecular modeling of RHD of RPGR to RCC1 shows that all disease mutations in RHD map to a distinct contact interface from that found between RCC1 and RAN GTPase. Two‐hybrid assays show that disease‐causing mutations in RHD or ORF15 domains differentially impair RPGR interaction with RPGRIP1. Cell and time‐lapse microscopy assays support that expression of RPGRIP1α1 isoform alone promotes the genesis of aggregates, whereas its co‐expression with the RPGR1–19 isoform targets RPGRIP1α1 to the Golgi. Conversely, RPGRORF15 co‐expression with RPGRIP1α1 promotes its pan intracellular dispersion and clears out pre‐existing RPGRIP1α1 deposits. Mutations singly in Rpgr1–19, RpgrORF15 or Rpgrip1, do not affect the colocalization of RPGR1–19 or RPGRORF15 with RPGRIP1α1, whereas co‐expression of similar mutations in Rpgr variants and Rpgrip1 abrogates their colocalization and association. RPGRORF15, but not RPGR1–19, prevents the C‐terminal proteolytic processing of RPGRIP1α1. Hence, Rpgr1–19 and RpgrORF15 exert distinct effects on RPGRIP1α1 sorting and processing and they are necessary but not sufficient to the coupling to, and subcellular targeting of, RPGRIP1α1.NIH support GM083165 & EY019492