Unwrapping the ciliary coat: high-resolution structure and function of the ciliary glycocalyx
The glycocalyx, a highly heterogeneous glycoprotein layer of cilia regulates adhesion and force transduction, and is involved in signalling. The high-resolution molecular architecture of this layer is currently not understood. We describe the structure of the ciliary coat in the green alga Chlamydomonas reinhardtii by cryo-electron tomography and proteomic approaches and present the high-resolution cryoEM structure of the main component, FMG1B. We describe FMG1B as a mucin orthologue which lacks the major O-glycosylation of mammalian mucins, but is N-glycosylated. FMG1A, a previously undescribed isoform of FMG1B is expressed in C. reinhardtii. By microflow-based adhesion assays we observe increased surface adhesion in the glycocalyx deficient double-mutant fmg1a-fmg1b. We find this mutant to be capable of surface-gliding, with neither isoform required for extracellular force transduction by intraflagellar transport. Our results find FMG1 to form a protective layer with adhesion-regulative instead of adhesion-conferring properties and an example of an undescribed class of mucins.