Gliding motility in Aphanothece halophytica: analysis of wall proteins in mot mutants

The unicellular cyanobacterium Aphanothece halophytica (PCC 7418) is motile, and spontaneous nonmotile (mot) mutants accumulate when the organism is subcultured. Analysis of mot mutants suggests that a glycoprotein in the cell wall is involved in the motility mechanism. Proteins from the wall fraction of the wild type and five mot clones were analyzed by gradient sodium dodecyl sulfate-acrylamide gel electrophoresis. Four clones were similar to the wild type, and one clone, mot-3, was missing a high-molecular-weight protein (approximately 200,000) and had at least one new polypeptide (160,000). The high-molecular-weight protein stained with periodic acid-Schiff reagent, suggesting that it was a glycoprotein. The absence of the protein in mot-3 did not affect the mechanical strength of the wall, since both mot-3 and wild-type cells were broken at the same rate by controlled cavitation. Several other cyanobacteria were also screened for the presence of glycoproteins. All motile strains have such proteins, although none had an apparent molecular weight as high as that in Aphanothece sp. Some motile strains, such as Oscillatoria limnetica and Phormidium sp., showed very large amounts of glycoproteins; whereas some nonmotile strains, such as Synechococcus sp. (UTEX 625) and Microcystis sp. (PCC 7820), showed no high-molecular-weight glycoproteins.