The dimensions of the protein import channels in the outer and inner mitochondrial membranes

Most mitochondrial proteins are imported into mitochondria through transmem brane channels composed largely, and perhaps exclusively, of proteins. We h ave determined the effective internal diameter of the protein import channe l in the mitochondrial outer membrane to be between 20 Angstrom and 26 Angs trom during translocation. The diameter of the import channel in the inner membrane is smaller than the diameter of the outer membrane import channel. These results were obtained by measuring the effect of rigid steric bulk i ntroduced into precursor proteins on import.