Chaperones are essential proteins whose role is to help the folding of other proteins as well as the transfer of poorly soluble proteins to the intracellular location where they fulfill their functions. Chaperones thus help proteins in their maturation by preventing the formation of aggregates primarily by binding the hydrophobic, aggregation-prone parts of their “cargo”, using hydrophobic patches on the chaperone surface. These hydrophobic interactions lead to a promiscuous "chaperone-client" complex. At the same time, this interaction must not be too strong to allow the two proteins to separate, especially when the complex reaches its final destination. In addition, chaperones must maintain a balance between promiscuity, which allows them to transport a wide variety of proteins, and specificity, which helps organize the final location of their "clients". Our knowledge of how interactions enable this balance between promiscuity and client specificity is limited.

The mitochondrion is an organelle surrounded by a two membranes (i.e. two phospholipid bilayers). The human mitochondrial proteome is estimated to contain more than a thousand proteins, 99% of which must be imported inside the mitochondria, i.e. in one of the mitochondrial membranes, in the intermembrane space, in the matrix or in very specific locations. The molecular mechanisms of this import are still poorly understood and involve chaperones.
A study by IRIG researchers has made it possible to decipher the specific mechanism of the chaperone system present in the intermembrane space of the mitochondria. Their study highlighted how TIM8-13 and TIM9-10, two homologous chaperones but with different functions, interact with two different membrane proteins of the inner mitochondrial membrane, namely Tim23 and mitochondrial-carrier proteins, which allow the mitochondrial matrix to exchange molecules with its surrounding. By combining NMR, small angle X-ray scattering (SAXS), analytical ultracentrifugation and molecular dynamics simulation techniques with other biophysical/biochemical approaches, the researchers were able to study the structures of the TIM8-13 and TIM9-10 chaperones in complex with different membrane proteins. They revealed that the delicate balance between promiscuity and specificity that these chaperones must satisfy is the result of a combination of a multitude of hydrophobic and hydrophilic interactions towards different client proteins.


The TIM chaperone (green/blue) carries a membrane protein (orange) to its final destination in the inner membrane of the mitochondria. The hydrophobic interactions between the chaperone cavity (bottom) and the hydrophilic interactions with the more polar part of the membrane protein ("N-tail") establish a highly dynamic complex, which is exchanged between multiple states. This flexibility also allows the membrane protein to be released for insertion into the membrane.