ABSTRACT
Ignicoccus is the only archaeal genus known today whose cells possess an outer membrane. According to freeze-etch experiments, it is composed of two leaflets which become separated in the fracture process. Here we show by transmission electron microscopy that the two leaflets can also be visualized in ultrathin sections; they exhibit highly different staining intensities. Biochemical analysis proves the presence of lipids as well as membrane proteins. Various derivatives of the archaeal lipid "archaeol" could be identified, many of which were glycosylated. The protein set is dominated by four membrane proteins, one or several of which may form pores. The outer membrane itself is a dynamic structure: periplasmic vesicles can be visualized in various stages of a fusion process, and, although rarely, vesicles are seen on the outer cell surface, either in a release or a fusion process. Future studies will focus on the outer membrane proteins in order to understand their role in outer membrane permeability, e.g. what kinds of transport processes they facilitate.
