Oscillin, an extracellular, Ca²⁺-binding glycoprotein essential for the gliding motility of cyanobacteria

Electron microscopic studies have demonstrated that various gliding filamentous cyanobacteria have trichome surfaces with a common structural organization. They contain an S-layer attached to the outer membrane and an array of parallel fibrils on top of the S-layer. In all species studied, the helical arrangement of these fibrils corresponds to the sense of rotation of the organism during the gliding movement. We have investigated the surface fibrils of Phormidium uncinatum using electron microscopic, spectroscopic and biochemical techniques. The fibrils consist of a single rod-shaped protein, which we refer to as oscillin. Oscillin is a 646 amino acid residue protein (M(r) 65 807; pI 3.63) and appears to be glycosylated. Sequence analysis reveals a two-domain structure: a 554 residue domain contains 46 repeats of a Ca²⁺-binding motif; it is followed by a 92 residue C-terminal domain, which might mediate its export. Filaments that do not express oscillin lose their ability to move. Homology studies suggest that similar proteins play comparable roles In other motile cyanobacteria. The structure of oscillin appears to favour a passive role in gliding.