Unwrapping the Ciliary Coat: High‐Resolution Structure and Function of the Ciliary Glycocalyx

TL;DR

This paper reveals the high-resolution structure and function of the ciliary glycocalyx in Chlamydomonas reinhardtii, identifying FMG1 as a new class of mucin-like proteins involved in adhesion regulation.

Contribution

The study identifies FMG1 as a novel mucin-like protein family and elucidates its role in the ciliary glycocalyx structure and adhesion regulation.

Findings

FMG1B is N-glycosylated but lacks O-glycosylation typical of mammalian mucins.

FMG1A, a new isoform of FMG1B, is expressed in Chlamydomonas reinhardtii.

The fmg1b-fmg1a double-mutant shows increased surface adhesion and surface-gliding ability.

Abstract

The glycocalyx, a highly heterogeneous glycoprotein layer of cilia regulates adhesion and force transduction and is involved in signaling. The high‐resolution molecular architecture of this layer is currently not understood. The structure of the ciliary coat is described in the green alga Chlamydomonas reinhardtii by cryo‐electron tomography and proteomic approaches and the high‐resolution cryoEM structure of the main component, FMG1B is solved. FMG1B is described 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, increased surface adhesion in the glycocalyx deficient double‐mutant fmg1b‐fmg1a is observed. It is found this mutant is capable of surface‐gliding, with neither isoform required for extracellular force transduction by intraflagellar transport. The 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.

The ciliary membrane is decorated in glycosylated proteins that define the interaction of the cilium with its environment. The main component of the ciliary coat of the green alga Chlamydomonas reinhardtii, FMG1, is characterized by cryo‐electron microscopy, proteomics, and live cell microscopy with flow‐based adhesion assays. FMG1 is described as a member of a new family of mucin‐like proteins.

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