RIBOSOME BINDING TO MITOCHONDRIA IS REGULATED BY GTP AND THE TRANSIT PEPTIDE

The association between ribosomes and the pore proteins at the endopla smic reticulum membrane is important to co-translational translocation . To determine if a similar association occurs between the ribosome an d mitochondrial membrane protein(s) during protein import in higher eu karyotes, we examined ribosome-mitochondria binding. By using spectral measurements, analysis of mitochondrial associated RNA, and electron microscopy, we demonstrated that ribosomes stably bind to purified rat liver mitochondria in vitro. Binding of ribosomes to mitochondria was markedly reduced by GTP and nearly abolished by the non-hydrolyzable GTP analogue, guanosine-5'-[thio]-triphosphate (GTP gamma S), but was only modestly reduced by GDP or ATP and unaffected by CTP. The initial rate of GTP hydrolysis by mitochondria was increased by ribosomes, wh ereas the rate of ATP hydrolysis by mitochondria was not affected. Rib osomes programmed with mRNA for 92 amino acids of the N terminus of mi tochondrial malate dehydrogenase bound to mitochondria, but unlike unp rogrammed rat liver ribosomes, neither GTP nor GDP disrupted binding; however, GTP gamma S did. These data show that receptors specific for ribosomes are present on the mitochondrial membrane, and a GTP-depende nt process mediates this binding. The presence of a nascent chain alte rs these binding characteristics. These findings support the hypothesi s that a co-translational translocation pathway exists for import of p roteins into mitochondria.