Analysis of ER Resident Proteins in Saccharomyces cerevisiae: Implementation of H/KDEL Retrieval Sequences

An elaborate quality control system regulates endoplasmic reticulum (ER) homeostasis by ensuring the fidelity of protein synthesis and maturation. In budding yeast, genomic analyses and high‐throughput proteomic studies have identified ER resident proteins that restore homeostasis following local perturbations. Yet, how these folding factors modulate stress has been largely unexplored. In this study, we designed a series of polymerase chain reaction (PCR)‐based modules including codon‐optimized epitopes and fluorescent protein (FP) variants complete with C‐terminal H/KDEL retrieval motifs. These conserved sequences are inherent to most soluble ER resident proteins. To monitor multiple proteins simultaneously, H/KDEL cassettes are available with six different selection markers, providing optimal flexibility for live‐cell imaging and multicolor labeling in vivo. A single pair of PCR primers can be used for the amplification of these 26 modules, enabling numerous combinations of tags and selection markers. The versatility of pCY H/KDEL cassettes was demonstrated by labeling BiP/Kar2p, Pdi1p and Scj1p with all novel tags, thus providing a direct comparison among FP variants. Furthermore, to advance in vitro studies of yeast ER proteins, Strep‐tag II was engineered with a C‐terminal retrieval sequence. Here, an efficient purification strategy was established for BiP under physiological conditions.