LIG_deltaCOP1_diTrp_1

Eukaryotic cells contain many different organelles between which vesicles traffic cargo material. COPI-coated vesicles mediate essential, evolutionarily conserved retrograde trafficking pathways from the Golgi to the endoplasmic reticulum (ER) and within the Golgi. The COPI vesicle coat consists of two major components, coatomer and the small GTPase ADP ribosylation factor 1 (Arf1). Coatomer is a ∼600 kDa heteroheptameric complex consisting of two linked subcomplexes, the βγδζ-COP F-subcomplex and the αβ’ε-COP B-subcomplex, all conserved from yeast to humans (Suckling,2015). The life cycle of COPI-coated vesicles is controlled by essential assembly/disassembly factors. These include their specific multisubunit tethering complexes, SNARE complexes and the regulators of Arf1, the ArfGAPs. In the final stages of their life, COPI-coated vesicles dock with the ER via the Dsl1-tethering complex, complete uncoating, and finally undergo SNARE-mediated fusion with the ER membrane (Ren,2009; Suckling,2015).
The three-subunit yeast Dsl1 tethering complex forms an approximately 20 nm tall tower, with the Sec39 and Tip20 subunits together forming the base of the tower, interacting directly with two different ER SNAREs. At the tip of the tower is Dsl1, which contains a large, disordered lasso-like loop that can reach far from the ER membrane to capture COPI vesicles (Ren,2009). The interaction between tryptophan-based motifs surrounded by negatively charged residues within this lasso-like loop of Dsl1 with coatomer subunits δ and α (Reilly,2001; Andag,2003; Suckling,2015) have an important role in vesicle tethering.
In yeasts, the C-terminal μ homology domain (MHD) of δ-COP is responsible for recognising the tryptophan-based motifs of the Dsl1 lasso (5FJZ; Suckling,2015). This interaction does not seem to be preserved in mammals, as the mammalian Dsl1 homolog ZW10 lacks a tryptophan-rich lasso. However, the δ-COP MHD domain was also reported to interact with representatives of another class of COPI accessory factors, the ArfGAPs, Gcs1p in yeast (5FJX; Suckling,2015), and ArfGAP1 in mammals (Rawet,2010; Suckling,2015) through their C-terminal tryptophan motifs.
Based on the presence of similar Trp motifs in ER resident proteins, other potential interaction partners of δ-COP MHD were also proposed (Cex1p and Sec66 in yeast), however, the functional relevance of their potential association remains to be elucidated (Cosson,1998). Initially, the tryptophan-based motifs were proposed to function as ER localization/retrieval determinants (Cosson,1998).
The MHD of δ-COP is unique among other MHDs of vesicle coat adaptors in many regards: 1) While in clathrin-coated vesicle formation MHDs of clathrin adaptor AP complexes bind motifs within membrane-embedded cargos, the MHD of δ-COP mediates crucial interactions with vesicle tethering/(dis)assembly factors, which affect the life cycle of COPI vesicles. 2) Binding of di-tryptophan motifs is unique to δ-COP MHD because the residues and the lengths of loops involved in binding are not conserved in the MHDs of AP complexes or the TSET complex. Also, binding of the di-Trp motifs occurs at a site of δ-COP MHD that is completely different from the binding site of YxxΦ cargo motifs on AP MHDs (Suckling,2015). 3) A striking feature of the δ-COP MHD is its large negative electrostatic surface potential, which is conserved across species and is therefore presumably functionally important but is in marked contrast to the MHDs of AP1 and especially AP2, which are extremely positively charged. Since δ-COP MHD is actually on the outside of the coat, being the furthest part of the COPI coat from the membrane (Dodonova,2015), its high negative charge may contribute to the specificity of target membrane fusion of COPI-coated vesicles. It could inhibit close apposition and hence fusion with more negatively charged membranes such as the late Golgi/TGN, but not inhibit contact with the comparatively uncharged membranes of the cis-Golgi and ER (Suckling,2015). 4) EM tomography suggests that the δ-COP MHD motif binding site remains accessible in the hyper-open membrane-associated conformation, as well as in the assumed closed conformation of the coatomer F-subcomplex, indicating that it lacks a conformational switching mechanism to regulate ligand binding, like AP complexes. This again likely reflects the markedly different roles of the MHDs in COPI and AP complexes (Suckling,2015).