Microsphere attachment induces glycoprotein redistribution and transmembrane signaling in theChlamydomonas flagellum

The biflagellate green algaChlamydomonas can exhibit substrate-associated gliding motility in addition to its ability to swim through a liquid medium. The flagella are the organelles responsible for both forms of whole-cell locomotion although the mechanism in each case is very different. In this study, we demonstrate that the binding of polystyrene microspheres to the flagellar surface ofChlamydomonas initiates clustering of the major flagellar-membrane glycoprotein, which is known to be involved in motility-associated substrate adhesion. In addition, we demonstrate that microsphere binding to the flagellar surface initiates the same transmembrane signaling pathway that is initiated by antibody- or lectin-induced crosslinking of the major flagellar-membrane glycoprotein. In each case, the signaling pathway involves the activation of a calciumdependent protein phosphatase that dephosphorylates a flagellar phosphoprotein known to be associated with the major flagellar-membrane glycoprotein. Bound microspheres are translocated along the flagellar surface at approximately the same velocity as whole-cell gliding motility. Previous observations suggest that microsphere binding and translocation along the flagellar surface may be a reflection of the same force-transducing system responsible for whole-cell gliding motility. In which case, these observations suggest that the transmembrane signaling pathway initiated by crosslinking the major flagellar-membrane glycoprotein is the same one that is activated when the cell contacts a physiological substrate by its flagellar surface.