The role of the exocyst member EXOC8 in the formation of cytoplasmic rods and rings

Abstract

Inosine monophosphate dehydrogenase (IMPDH) and cytidine triphosphate synthase (CTPS) are two rate-limiting enzymes involved in the catalysis of GTP and CTP. In humans, these enzymes can form into filamentous structures termed cytoplasmic rods and rings. Although cytoplasmic rods and rings have been described in multiple cell types, neither their formation nor their composition are well understood. Here we identify a novel localisation for the exocyst complex subunit EXOC8 to cytoplasmic rod/ring structures. Upon closer inspection, these structures resemble beads on a string, similar to the structure of IMPDH2-containing cytoplasmic rods and rings previously observed by cryo-EM. We found that EXOC8 interacts physically with IMPDH2 and CTPS1. Although it is not required to form IMPDH2-containing rods and rings in non-stressed cells, when these structures are induced by changes to metabolic conditions, rod length and organisation is altered in the absence of EXOC8. Mutations in the closely related enzyme IMPDH1 are a cause of inherited retinal degeneration, and mutation in EXOC8 is a candidate for pleiotropic syndromes where retinal dystrophy is part of the clinical presentation. We found that rod and ring localisation was lost in a disease-linked EXOC8 variant, and interaction with IMPDH2 was lost. This is the first time that an exocyst subunit - and indeed any membrane trafficking complex - has been linked to cytoplasmic rods and rings. We propose that the exocyst plays a role in trafficking proteins required for IMPDH2 rod and ring maturation and/or turnover, thereby regulating cyclic nucleotide metabolism, and that dysregulation may contribute to the molecular pathology of retinal degeneration and phototransduction.