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.