innermembranprotein

Inner membrane proteins (IMPs) are a broad class of polypeptides localized to the inner membrane of bacteria and organelles such as mitochondria and chloroplasts. They are embedded in the lipid bilayer and perform a wide range of essential cellular functions, often with multiple transmembrane helices.

Localization and topology of IMPs is defined by their hydrophobic transmembrane segments. Their topology describes how many membranes they traverse and the orientation of their termini, such as cytoplasmic versus matrix sides. Many IMPs feature 3 to 20 helices and may have periplasmic or matrix-facing domains that contribute to their function and regulation.

Functions of inner membrane proteins are diverse. In bacteria, the inner membrane hosts components of the respiratory chain, various transporters, and secretion or insertion systems. In mitochondria, inner membrane proteins include the subunits of respiratory complexes I–IV, ATP synthase, and metabolite transporters, as well as protein-import machineries such as TIM23 and TIM22. The inner membrane also participates in maintaining the membrane potential and shaping cristae structure, which are important for energy production and metabolite exchange.

Biogenesis and targeting of IMPs differ across organisms. Bacterial IMPs are inserted by the Sec translocon, the YidC insertase, or related pathways. In mitochondria, most nuclear-encoded IMPs are synthesized in the cytosol and imported via the TOM complex, followed by insertion into the inner membrane through TIM23 or TIM22 pathways; some inner membrane components are encoded by the mitochondrial genome and translated within the organelle.

Significance and research: IMPs are central to energy conversion, metabolite transport, and mitochondrial protein homeostasis. They are common antibiotic targets and are implicated in various diseases when malfunctions occur, making their study a focus of cell biology and biomedicine.