AbstractsBiology & Animal Science

Evolutionary Conservation in the Biogenesis of β-Barrel Proteins

by Thomas Ulrich

Institution: Universität Tübingen
Year: 2015
Record ID: 1114017
Full text PDF: http://hdl.handle.net/10900/59809


The vast majority of outer membrane proteins (OMPs) in Gram-negative bacteria belong to the class of membrane-embedded β-barrel proteins. Besides Gram-negative bacteria, the presence of these proteins is restricted to the outer membranes (OM) of the eukaryotic organelles mitochondria and chloroplasts. This can be seen as evidence for the endosymbiotic theory, according to which these organelles derived from the engulfment of prokaryotic ancestors into a progenitor of the eukaryotic cell. The process of organellogenesis led to a major DNA transfer of genes encoding mitochondrial proteins to the host genome. As a consequence, more than 99% of the proteins in present-day mitochondria are encoded in the nucleus and thus have to contain all the information required for specific sorting to their target compartment within mitochondria. Although progress has been made in understanding the targeting signals in numerous mitochondrial proteins, the signal that ensures targeting of β-barrel proteins still remains elusive. It is remarkable that in all membranes harboring these proteins the assembly is facilitated by dedicated protein complexes that contain a highly conserved central β-barrel protein termed BamA/YaeT/Omp85 in Gram-negative bacteria and Tob55/Sam50 in mitochondria. In this context it is astonishing that in spite of a very long divergent evolution of pro- and eukaryotes, mitochondria retained the ability to recognize and assemble bacterial β-barrel proteins. Due to this evolutionary conservation, yeast mitochondria can provide us a useful model system to study the biogenesis of β-barrel proteins from different origin. Currently, little is known about the signal that ensures specific targeting of β-barrel proteins to either mitochondria or chloroplasts. To shed light on this topic, I investigated the targeting of the chloroplast β-barrel proteins Oep24 and Oep37 upon their expression in yeast cells. We could demonstrate their exclusive localization to the mitochondrial outer membrane (MOM). Assembly of Oep37 partially complemented the growth phenotype of yeast cells lacking the general metabolite transporter Porin. Interestingly, both proteins followed in yeast cells a pathway similar to the one undertaken by bona fide mitochondrial β-barrel proteins. In another part of my studies I investigated targeting of the trimeric autotransporter protein Yersinia adhesin A (YadA). Specifically, I was interested in the mechanism by which precursors of such proteins cross the periplasm and assemble in the OM. To that goal, we took advantage of the evolutionary conservation in the biogenesis of β-barrel proteins between bacteria and mitochondria. Upon expression in yeast cells, both monomeric and trimeric forms of YadA were targeted to mitochondria, but solely the trimeric form was fully assembled into the MOM. Remarkably, the co-expression of YadA with a mitochondrially-targeted form of the bacterial periplasmic chaperone Skp, but not with SurA or SecB, resulted in elevated levels of YadA. Taken together, the results…