Axonemal localization of Chlamydomonas PACRG, a homologue of the human Parkin-coregulated gene product

A homologue of mammalian PACRG was identified in Sarkosyl-extracted Chlamydomonas axonemes as a protein that may interact with Rib72 (a component of the protofilament ribbon within the outer doublet microtubules). PACRG is a protein whose expression is co-regulated with the Parkin gene implicated in Parkinson's disease. Although subsequent analyses did not confirm a Rib72-PACRG interaction, both proteins display similar localization in the axoneme. Immuno-localization of PACRG required pretreatment of the axoneme with Sarkosyl, suggesting that the antigen is buried in the wall of the microtubule. Indirect immunofluorescence localized PACRG to the entire length of the axoneme and the basal body, and immuno-electron microscopy showed that the PACRG antigen is densely distributed along the outer doublets in frayed axonemes. In thin-section images, the PACRG signals were frequently found between the A- and B-tubules of adjacent outer doublets. From these and other results, we propose that PACRG is a structural component of the doublet and triplet microtubules possibly involved in inter-tubule linkage.