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bioRxiv2019

Crystal structure of human PACRG in complex with MEIG1

In human, the Parkin Co-Regulated Gene (PACRG) shares a bidirectional promoter with Parkin, a gene involved in Parkinsons disease, mitochondrial quality control and inflammation. The PACRG protein is essential to the formation of the inner junction between doublet microtubules of the axoneme, a structure found in flagella and cilia. PACRG interacts with tubulin as well as the meiosis expressed gene 1 (MEIG1) protein, which is essential for spermiogenesis in mice. However, the 3D structure of human PACRG is unknown. Here, we report the crystal structure of the C-terminal domain of human PACRG in complex with MEIG1 at 2.1 [A] resolution. PACRG adopts an -helical structure with a loop insertion that mediates a conserved network of interactions with MEIG1. Using the cryo-electron tomography structure of the axonemal doublet microtubule from the flagellated protozoan Chlamydomonas reinhardtii, we generate a model of a mammalian microtubule doublet inner junction, which reveals how PACRG interacts with tubulin subunits in both the A- and B-tubules. Furthermore, the model shows that MEIG1 interacts with {beta}-tubulin on the outer surface of the B-tubule, facing towards the central pair of the axoneme. We also model the PACRG-like protein (PACRGL), a homolog of PACRG with potential roles in microtubule remodelling and axonemal inner junction formation. Finally, we explore the evolution of the PACRG and Parkin head-to-head gene structure and analyze the tissue distribution of their transcripts. Our work establishes a framework to assess the function of the PACRG family of proteins and its adaptor proteins in the function of motile and non-motile cilia.

cell biology↗

bioRxiv2019

The inner junction complex of the cilia is an interaction hub that involves tubulin post-translational modifications

Microtubules are cytoskeletal structures involved in structural support, microtubule-based transport and the organization of organelles in the cells. The building blocks of the microtubule, the - and {beta}-tubulin heterodimers, polymerize into protofilaments, that associate laterally to form the hollow microtubule. There exists a specific type of microtubule structures in the cilia, termed doublet microtubules, where high stability is required for ciliary beating and function. The doublet microtubule, consisting of a complete A-tubule and a partial B-tubule maintains its stability through unique interactions at its outer and inner junctions, where the A- and B-tubules meet.\n\nUsing cryo-electron microscopy, we present the answer to the long-standing question regarding the identities, localizations and structures of the Chlamydomonas doublet microtubule inner junction proteins. Using a combination of sequence bioinformatics and mass spectrometry, we identified two new inner junction proteins, FAP276 and FAP106, and an inner junction associated protein FAP126. We show that inner junction proteins PACRG and FAP20, together with FAP52, previously unidentified FAP276, FAP106 and FAP126, form an interaction hub at the inner junction, which involves tubulin sites for post-translational modifications. We further compare the Chlamydomonas and Tetrahymena doublet microtubule structures to understand the common and species-specific features of the inner junction.

biophysics↗