An actomyosin-like cytoskeleton in the cyanobiont (Nosctoc sp.) of Peltigera canina

Abstract

Lichenized Nostoc cells isolated from the lichen Peltigera canina develop chemotaxis towards a lectin purified from the lichen thallus. Similar movements in unicellular eukaryotes require actin and myosin to generate contraction and relaxation along the chemotactic axis. We provide evidences for prokaryotic actin-like and myosin-like proteins in the cyanobiont Nostoc sp using cross-reacting antibodies against α- and β-actin and non-muscle myosin II light and heavy chains, and two-dimensional gel electrophoresis to determine the isoelectric point (IP) of the actin-like protein. Actin antibodies bound to a single reactive Nostoc polypeptide of an approximate molecular mass of 50 kD with IP values between 4 and 7 pH, similar to eukaryotic actin. The myosin light chain antibody reacted with a Nostoc protein with an apparent molecular weight of 20 kDa and another of 48 kDa. Immunoprecipitation of cell free extracts using anti-heavy chain myosin separately yielded only one signal corresponding to a protein of a molecular weight around 200 kDa. A bioinformatics analysis indicated that (cyano)bacterial actins are rare but do exist. Our results are consistent with the possible existence of protein homologues of actins and myosins in cyanobionts of P. canina, suggesting the existence of an actin-like apparatus that supports chemotactic swimming.