Abstract
The emergence of an endomembrane system was a crucial stage in the prokaryote-to-eukaryote evolutionary transition. Recent genomic and molecular evolutionary analyses have provided insight into how this critical system arrived at its modern configuration. The apparent relative absence of prokaryotic antecedents for the endomembrane machinery contrasts with the situation for mitochondria, plastids and the nucleus. Overall, the evidence suggests an autogenous origin for the eukaryotic membrane-trafficking machinery. The emerging picture is that early eukaryotic ancestors had a complex endomembrane system, which implies that this cellular system evolved relatively rapidly after the proto-eukaryote diverged away from the other prokaryotic lines. Many of the components of the trafficking system are the result of gene duplications that have produced proteins that have similar functions but differ in their subcellular location. A proto-eukaryote possessing a very simple trafficking system could thus have evolved to near modern complexity in the last common eukaryotic ancestor (LCEA) via paralogous gene family expansion of the proteins encoding organelle identity. The descendents of this common ancestor have undergone further modification of the trafficking machinery; unicellular simplicity and multicellular complexity are the prevailing trend, but there are some remarkable counter-examples.
Highlights
Eukaryotic cells differ fundamentally from their prokaryotic counter-parts by their possession of internal, membrane-bound, compartments
Much has been published about the evolution of mitochondria and plastids; the origin of the nucleus has received significant but less attention (Lopez-Garcia and Moreira, 2006; Martin, 2005)
Would have been the appearance of cellular machinery for internalization and digestion of extracellular material, targeted intracellular transport, surface remodelling and secretion. These functions are only possible in eukaryotic cells because of the presence of the membranetrafficking system
Summary
Eukaryotic cells differ fundamentally from their prokaryotic counter-parts by their possession of internal, membrane-bound, compartments. Protein factors including SNAREs, tethering complexes, syntaxin-binding proteins [ known as Sec1/Munc (SM) proteins], and Rab GTPases ensure specificity and fusion of these carriers with the correct target membrane (Jahn et al, 2003) Many of these protein families can be divided into subfamilies in which each paralogue performs a function similar to that of the other members but at a specific organellar location or in a distinct transport pathway (Bonifacino and Glick, 2004). The genes encoding these membrane-trafficking proteins are, amenable to molecular evolutionary analyses of the type that has yielded information on the emergence and history of the other eukaryotic organelles.
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