Conclusion/Perspective

In this concluding chapter the translocation machineries are compared, which are employed in different membrane systems to highlight their similarities and unique properties, depicting the Sec and Oxa1p translocation systems along with the Tat and peroxisomal pathway. This chapter puts forth a summary note by taking together the preceding chapters that described how proteins are transported across the bacterial plasma membrane, the endoplasmic reticulum (ER) membrane, the chloroplast thylakoid membrane, and how proteins are imported into mitochondria, chloroplasts, peroxisomes, and the nucleus. In chloroplast and bacteria, this is achieved by using the twin arginine translocation (Tat) translocase, comprising the TatA, TatB, TatC and TatE proteins. The Tat system is present in both the bacterial plasma membrane and the chloroplast thylakoid membrane. The Tom complex contains receptors that recognize the signal sequences of mitochondrial imported proteins. The field of protein targeting has come a long way since the pioneering studies by Günter Blobel and collaborators in the early 1970s. Literally, thousands of postdoc-years have gone into figuring out the complex targeting pathways and translocation mechanisms that have evolved to ship proteins within and between cells. And yet, we cannot claim to have a full, atomic-level understanding of even a single translocation process. Finally, this chapter concludes with a positive hope that there is certainly more to come!