Characterization of TSET, an ancient and widespread membrane trafficking complex.

Jennifer Hirst,Alexander Schlacht,Robin Antrobus,Robert R Kay,David Traynor,Gareth Bloomfield,Joel B Dacks,John P Norcott,Margaret S Robinson

doi:10.7554/elife.02866

Abstract

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.DOI: http://dx.doi.org/10.7554/eLife.02866.001.

Highlights

The evolution of eukaryotes some 2 billion years ago radically changed the biosphere, giving rise to most visible life on Earth
One of these proteins had been characterised functionally: TPLATE (NP_186827.2), an Arabidopsis protein related to the AP β subunits and β-COP, found in a microscopy-based screen for proteins involved in mitosis and localised to the cell plate (Van Damme et al, 2006; Van Damme et al, 2011)
There is some variability between orthologous subunits in different organisms: for instance, Arabidopsis has added an SH3 domain to the C-terminal end of its ‘γαδεζ’ large subunit, while Dictyostelium has lost the μ homology domain (MHD) at the end of its medium subunit; and in general there seems to be much less selective pressure on these genes than on those encoding other AP/COPI family members

Summary

Introduction

The evolution of eukaryotes some 2 billion years ago radically changed the biosphere, giving rise to most visible life on Earth. Key to this transition was the ability to generate intracellular membrane compartments and the trafficking pathways that interconnect them, mediated in part by the heterotetrameric adaptor complexes, APs 1–5 and COPI (Dacks et al, 2008; Field and Dacks, 2009; Hirst et al, 2011; Koumandou et al, 2013). We set out in search of additional members of the AP/COPI subunit families

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Conclusion