Abstract 259 12.1 Introduction 260 12.2 TOM and TIM Translocases of Mitochondria 261

12.2.1 TOM Translocase 261 12.2.2 TIM Translocases (TIM23 and TIM22) 261 12.2.3 Origin of TOM and TIM Translocases 2632

12.3 TOC and TIC Translocases of Chloroplasts 263 12.3.1 TOC Translocase 264 12.3.2 The TIC Translocase 265 12.3.3 Origin of TOC and TIC Translocases 268

12.4 Intraorganellar Protein Translocation 268 12.4.1 The Sec Pathway 269 12.4.2 The Tat Pathway 271 12.4.3 Signal Recognition Particle (SRP) Pathway 273 12.4.4 YidC/Oxa1p/Alb3 Translocase Family 275

Acknowledgment 276 References 276

Mitochondria and chloroplasts originated from free-living bacteria as a result of endosymbiosis. After the endosymbiotic event, most of the prokaryotic genes were lost or transferred to the host nucleus (see Chapter 11). The nuclear-encoded organellar proteins are synthesized as precursors with an amino-terminal cleavable presequence, which carries targeting and sorting information. The outer and inner membranes of mitochondria contain the TOM and TIM complex, respectively, which are responsible for selective protein translocation across the membranes. The outer and inner membranes of chloroplasts contain functional equivalents, the TOC and TIC complexes, respectively. The main subunits of these complexes have been identified and characterized in vitro and in vivo. Some subunits of the TOC and

TIC complexes have homologues in cyanobacteria, suggesting that the chloroplast protein import machinery might have been built around a prokaryotic core. In contrast, so far there is no report on bacterial proteins that share significant homology to the subunits of the TOM and TIM complexes. Translocation into or across the thylakoid membranes of chloroplasts is catalyzed by the Sec, the signal recognition particle (SRP), the pH-dependent or Tat and the YidC/Oxa1p/Alb3 pathways. Translocation of proteins into or across the inner membrane of mitochondria also involves SRP components and Oxa1p. Phylogenetic evidence suggests that these intraorganellar pathways are derived from preexisting prokaryotic systems for protein export.