Subfractional analysis of cyanobacterial membranes and isolation of plasma membranes by aqueous polymer two-phase partitioning.

The present work demonstrates that partition in aqueous polymer two-phase systems offers a rapid method for separation and isolation of thylakoid, plasma, and outer membranes from cyanobacteria. Pure plasma membranes from Phormidium laminosum can be isolated by this method within 3 h, starting with total membranes obtained by French press treatment of the cyanobacterial cells. The isolated plasma membranes have a broad density profile, giving rise to three subpopulations. The main fraction has the same density as the abundant thylakoid membranes. This fraction has not been resolved in previous separations based on sucrose gradient centrifugation, which is the only method previously used for isolation of cyanobacterial plasma membranes. Another advantage of the aqueous polymer two-phase system is that it can handle large quantities of starting material, which is essential to obtain a satisfactory yield since plasma membranes constitute only a very small fraction of the total membrane content in a cyanobacterial cell. The isolation procedure results in a pure plasma membrane preparation with retained cytochrome c oxidase activity. The results also point to the possibility of a lateral heterogeneity in the organization of the cyanobacterial plasma membrane.