Insertion of model helices into the mitochondrial inner membrane: the rules of the game

Because of their hydrophobic and hydrophilic nature and the need for a lipid bilayer for retaining the native structure, membrane proteins are hard to study. Nevertheless, they are important, as many of our diseases are related to membrane proteins and around 60% of the different pharmaceutical drugs are directed against a membrane proteins [1]. There are many ways to study a protein, you can study function, structure, how the protein is targeted and inserted into its specific organelle, the interactions with other proteins or ligands etc. In the absence of a high-resolution structure, a topology model for a membrane protein is often useful. We have obtained reliable topologies for 546 of the membrane proteins going through the secretory pathways in S. cerevisiae by combining experimental data with topology prediction programs. In addition we have produced topology models for over 15,000 membrane proteins from 38 sequenced eukaryotic genomes using homology to the experimentally determined group. We also examined the growth rates and tolerance to certain stress conditions for our large set of clones that over-express membrane proteins. This provides important information both for structural studies of membrane proteins where large amounts of protein is needed for further studies, and for getting some insight in the function of specific proteins. Finally we have studied the integration of membrane proteins by the Tim23 translocon in the inner membrane of mitochondria. We have investigated the hydrophobicity required for efficient integration of transmembrane (TM) helices by Tim23. From this data we have derived an in vivo hydrophobicity scale for the insertion of different amino acids into the inner membrane of the mitochondria, and have made a comparison with a previously determined hydrophobicity scale for the ER translocon Sec61. We concluded that charged residues flanking the TM segment are of major importance for insertion into the membrane. We therefore further investigated the importance of charged residues flanking the first, weakly hydrophobic, TM segment in the mitochondrial inner membrane protein Mgm1p with regard to membrane insertion by the Tim23 complex.