Proteins that are moving from the endoplasmic reticulum to the Golgi apparatus travel inside of COPII-coated transport vehicles. According to the fourth edition of Molecular Biology of the Cell on the NCBI website, the transport vesicles are pinched off from a region of the endoplasmic reticulum known as ER exit sites. These exit sites do not have membrane-bound ribosomes that would otherwise retard the process.
Selected cargo proteins gravitate toward the transport vesicles, where they eventually become concentrated. Scientists believe these cargo proteins have an exit signal that alerts complementary receptor proteins to the presence of the COPII coat in the budding vesicle. This helps the complementary proteins move into the vesicle. Yet, some proteins without exit signals, which usually remain in the ER, also are packaged in budding vesicles to move into the Golgi apparatus. Approximately 200 membrane proteins, often of various types, are contained within the average 50 nanometer transport vesicle.
However, not all proteins require a vesicle. When secretory proteins are made in very high concentrations, some of them leak out of the ER without the help of sorting receptors. Other proteins, on the other hand, are not allowed to leave at all. For instance, when proteins are unfolded or misfolded, they are retained within the ER in a chaperone protein.