IDENTIFICATION OF 2 GTP-BINDING PROTEINS IN THE CHLOROPLAST PROTEIN IMPORT MACHINERY

Two of four proteins that associated with translocation intermediates during protein import across the outer chloroplast envelope membrane w ere identified as guanosine triphosphate (GTP)-binding proteins. Both proteins are integral membrane proteins of the outer chloroplast membr ane, and both are partially exposed on the chloroplast surface where t hey were accessible to thermolysin digestion. Engagement of the outer membrane's import machinery by an import substrate was inhibited by sl owly hydrolyzable or non-hydrolyzable GTP analogs. Thus, these GTP-bin ding proteins may function in protein import into chloroplasts.