insertases

Insertases are a class of membrane proteins that facilitate the insertion and assembly of hydrophobic integral membrane proteins into lipid bilayers. They aid the lateral insertion of transmembrane helices and, in many cases, help orient and stabilize newly inserted proteins during biogenesis. Insertases often work in coordination with other membrane machineries and chaperones, rather than transporting fully folded proteins across the membrane.

The best-known insertases are members of the YidC/Oxa1/Alb3 family. YidC operates in the bacterial inner membrane and can function independently or with the Sec translocon to insert a variety of membrane proteins, including some respiratory components. Oxa1 is the mitochondrial inner-membrane insertase in eukaryotes and participates in translating and inserting mitochondrial-encoded and imported nuclear-encoded proteins, often in cooperation with mitochondrial ribosomes. Alb3 is the chloroplast homolog that mediates insertion of thylakoid membrane proteins and collaborates with other chloroplast protein import and assembly systems. These proteins share structural features that create a hydrophilic groove or surface capable of engaging transmembrane segments and guiding them into the lipid bilayer.

Insertases are essential for the biogenesis of a wide range of membrane proteins, including subunits of respiratory chains and photosynthetic complexes. They can act as primary pathways for membrane protein insertion or as accessory factors that assist other translocation systems. Evolutionarily, YidC-like insertases are conserved across bacteria, mitochondria, and chloroplasts, reflecting an ancient mechanism for integrating membrane proteins into diverse organellar membranes.