Coat Protein I, often abbreviated as COPI, is a type of protein complex that plays a crucial role in the transport of proteins within cells. It is primarily involved in the retrograde transport of proteins from the Golgi apparatus back to the endoplasmic reticulum. This process is essential for maintaining the proper functioning and organization of cellular compartments. COPI is composed of several subunits that form a coat around vesicles, which are small membrane-bound sacs that transport materials. The assembly of COPI on these vesicles helps in their budding off from the Golgi and ensures that the correct proteins are delivered to their intended destinations within the cell.