The flagellar membrane glycoprotein FMG-1B Is necessary for expression of force at the flagellar surface

Abstract

In addition to bend propagation for swimming, Chlamydomonas cells use their flagella to glide along a surface. When polystyrene microspheres are added to cells, they attach to and move along the flagellar surface, thus serving as a proxy for gliding that can be used to assay for the flagellar components required for gliding motility. Gliding and microsphere movement are dependent on intraflagellar transport (IFT). Circumstantial evidence suggests that mechanical coupling of the IFT force-transducing machinery to a substrate is mediated by the flagellar transmembrane glycoprotein FMG-1B. Cells carrying an insertion in the 5’-UTR of the FMG-1B gene lack FMG-1B protein, yet assemble normal length flagella despite the loss of the major protein component of the flagellar membrane. Transmission electron microscopy shows a complete loss of the glycocalyx normally observed on the flagellar surface, suggesting it is composed of the ectodomains of FMG-1B molecules. Microsphere movements and gliding motility are also greatly reduced in the 5’-UTR mutant. Together, these data provide the first rigorous demonstration that FMG-1B is necessary for the normal expression of force at the flagellar surface in Chlamydomonas.