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Novel cargo-binding site in the ß and subunits of coatomer

¹ Zentrum für Molekulare Biologie der Universität Heidelberg (ZMBH), Im Neuenheimer Feld 282, D-69120 Heidelberg, Germany
² European Molecular Biology Laboratory (EMBL), Cell Biology Cell Biophysics Unit, D-69117 Heidelberg, Germany
³ Swiss Institute of Bioinformatics, Biozentrum, University of Basel, CH-4056 Basel, Switzerland
⁴ Growth & Development, Biozentrum, University of Basel, CH-4056 Basel, Switzerland

Correspondence to Blanche Schwappach: b.schwappach{at}zmbh.uni-heidelberg.de

Arginine (R)-based ER localization signals are sorting motifs that confer transient ER localization to unassembled subunits of multimeric membrane proteins. The COPI vesicle coat binds R-based signals but the molecular details remain unknown. Here, we use reporter membrane proteins based on the proteolipid Pmp2 fused to GFP and allele swapping of COPI subunits to map the recognition site for R-based signals. We show that two highly conserved stretches—in the ß- and -COPI subunits—are required to maintain Pmp2GFP reporters exposing R-based signals in the ER. Combining a deletion of 21 residues in -COP together with the mutation of three residues in ß-COP gave rise to a COPI coat that had lost its ability to recognize R-based signals, whilst the recognition of C-terminal di-lysine signals remained unimpaired. A homology model of the COPI trunk domain illustrates the recognition of R-based signals by COPI.

U. Liebel's present address is Institute of Toxicology and Genetics, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany.

Abbreviations used in this paper: AD, activation domain; BD, binding domain.

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