As disulfide bonds are added, newly generated membrane and secretory proteins in cells fold in the endoplasmic reticulum and acquire the proper three-dimensional shape. Disulfide bonds play a crucial role in protein folding. It has been hypothesised that a protein disulfide isomerase enzyme is primarily responsible for the production of protein disulfide bonds in eukaryotes. The endoplasmic reticulum (ER), a eukaryotic cellular organelle involved in protein synthesis, processing, and transport, has been thought to recycle proteins with the C-terminus of amino acids sequences containing His-Asp-Glu-Leu (HDEL) sequence in yeast. The analysis of a fusion protein with an HDEL tag led to the suggestion for this recycling mechanism. The investigation of protein disulfide isomerase's localization and oligosaccharide modification in yeast provided the first concrete proof that this intrinsic ER protein travels from ER to Golgi. According to the results, this native protein accumulates in the ER while being accessible to post-ER enzymes.