intermembrane

Intermembrane refers to the region between two biological membranes, most often the space between the outer and inner membranes enclosing certain organelles. The term is commonly used in reference to intermembrane spaces in mitochondria and chloroplasts, but it can also describe similar regions in other double-membrane systems; in bacteria, a related compartment is typically called the periplasm.

In mitochondria, the intermembrane space lies between the outer mitochondrial membrane and the inner mitochondrial membrane. It contains soluble proteins and small molecules that participate in respiration and signaling. The intermembrane space is connected to the cytosol via outer membrane channels, allowing diffusion of small ions and metabolites. It houses components of the respiratory chain released during outer membrane stress and serves as a reservoir of protons that contribute to the proton motive force across the inner membrane. Protein import into the intermembrane space occurs through the TOM and TIM translocase systems, and many IMS proteins are stabilized by the Mia40-Erv1 disulfide relay system, which facilitates oxidative folding. The IMS also plays a central role in apoptosis when cytochrome c is released into the cytosol, triggering caspase activation.

In chloroplasts, the intermembrane space is the region between the outer and inner envelope membranes. It participates in metabolite exchange and is involved in the targeting and transit of nuclear-encoded chloroplast proteins via the Toc/Tic translocon complexes, with many proteins ultimately localizing to the stroma. The intermembrane space contributes to maintaining distinct redox and ionic conditions relative to the cytosol and the chloroplast interior.

In bacteria, the analogous compartment between the inner cytoplasmic membrane and the outer membrane is usually termed the periplasm, not the intermembrane space. The periplasm contains enzymes for nutrient acquisition, electron transfer, and protein folding and maturation. See also intermembrane space, mitochondrion, chloroplast, periplasm.