Final answer:
The protein coat's soluble proteins assemble on the cytoplasmic (or inner) surface of the donor membrane during budding. These proteins are often peripheral proteins which can serve multiple purposes within the cell.
Step-by-step explanation:
The protein coat contains soluble proteins that assemble on the cytoplasmic surface of the donor membrane at sites where budding takes place. This place is essentially the inner surface of a cell's membrane. Proteins present on the exterior and interior surfaces of membranes are usually peripheral proteins, attached either to integral proteins or to phospholipids. Such proteins may serve as enzymes, structural attachments for the fibers of the cytoskeleton, or as part of the cell's recognition sites.
These are sometimes referred to as "cell-specific" proteins. The body can recognize its own proteins and fights against foreign proteins associated with pathogens. This process of protein assembly in the context of a cell's membrane is key to cell functionality, including aspects like the immune response, cell structure, and interactions between cells.
Learn more about Cytoplasmic Surface Protein Assembly