Fourmembrane

Fourmembrane is a term used in cell biology to describe structures or compartments that are enclosed by four lipid membranes. The concept is most often applied to plastids that arise through secondary endosymbiosis, in which a eukaryotic host cell engulfs a photosynthetic alga and retains multiple membranes from the endosymbiotic event.

In many fourmembrane plastids, the four envelopes consist of the two membranes of the original plastid plus two additional membranes contributed by the host during engulfment. This arrangement creates a periplastidial compartment between certain membranes and influences how proteins, metabolites, and ions are exchanged between the compartments.

Protein import into fourmembrane plastids is complex, frequently requiring multiple targeting signals and translocon systems that span more than one membrane. Metabolic processes may be distributed across compartments, and numerous plastid-related genes are relocated to the host nucleus, with the corresponding proteins reimported into the plastid.

Examples of fourmembrane plastids include the apicoplasts of Apicomplexa (such as Plasmodium) and various secondary plastids found in algae like diatoms and chlorarachniophytes. The precise number and arrangement of membranes can vary among lineages, and in some taxa the envelope count is described as three if one membrane is lost or fused during evolution.

See also: plastid, secondary endosymbiosis, apicoplast, chloroplast, endosymbiotic theory.