The heterotetrameric AP and F-COPI complexes help to define the cellular map of modern eukaryotes. To search for related machinery, we developed a structure-based bioinformatics tool, and identified the core subunits of TSET, a 'missing link' between the APs and COPI. Studies in Dictyostelium indicate that TSET is a heterohexamer, with two associated scaffolding proteins. TSET is non-essential in Dictyostelium, but may act in plasma membrane turnover, and is essentially identical to the recently described TPLATE complex, TPC. However, whereas TPC was reported to be plant-specific, we can identify a full or partial complex in every eukaryotic supergroup. An evolutionary path can be deduced from the earliest origins of the heterotetramer/scaffold coat to its multiple manifestations in modern organisms, including the mammalian muniscins, descendants of the TSET medium subunits. Thus, we have uncovered the machinery for an ancient and widespread pathway, which provides new insights into early eukaryotic evolution.
Eukaryotes make up almost all of the life on Earth that we can see around us, and include organisms as diverse as animals, fungi, plants, slime moulds, and seaweeds. The defining feature of eukaryotes is that, unlike nearly all bacteria, they have membrane-bound compartments—such as the nucleus—within their cells.
Moving molecules, such as proteins, between these compartments is essential for living eukaryotic cells, and these molecules are usually trafficked inside membrane-bound packages called vesicles. Two similar sets of protein complexes—each containing four different subunits—ensure that the molecules are packaged inside the correct vesicles. However, it is not clear how these two protein complexes (called the AP complexes and the COPI complex) are related to each other, and when and where they originated in the history of life.
Now, Hirst, Schlacht et al. have discovered a new—but very ancient–protein complex that they refer to as the ‘missing link’ between the AP and COPI complexes. The four subunits inside this new complex were found by searching for proteins with shapes that were similar to those of the AP and COPI proteins, rather than just searching for proteins with similar sequences of amino acids. This approach identified related protein subunits in groups as diverse as plants and slime moulds, which suggests that this protein complex evolved in the earliest of the eukaryotes. The four subunits identified in a slime mould were confirmed to interact, and also shown to bind to the plasma membrane of living cells.
One of the subunits had already been named TPLATE, so Hirst, Schlacht et al. decided to call the complex TSET; the other three subunits were named TSAUCER, TCUP and TSPOON, and two other proteins that interacted with the complex were both called TTRAY.
While most of the TSET complex itself has been lost from humans and other animals, one of subunit appears to have evolved into a family of proteins that help molecules get into cells. The discovery of TSET reveals another major player in vesicle-trafficking that is not only important for our understanding of how modern eukaryotes work, but also how ancient eukaryotes evolved.