In cell biology, bulk flow is the process by which proteins with a sorting signal travel to and from different cellular compartments. Proteins often have sorting signals, either transport signals, specifying if a protein will translocate to another compartment within the cell, or retention signals, specifying if it will be retained in the current, membrane-bound, compartment in which it is already located. For instance, proteins with the KDEL sorting signal are specified to return to the endoplasmic reticulum from the Golgi (see vesicular transport). However, proteins lacking a sorting signal will increase in concentration in a specific compartment until it reaches bulk concentration in the donor compartment. At this point, proteins enter budding vesicles and are transported to an acceptor compartment. This process is called bulk flow.
It is thought that cargo travels through the Golgi cisternae (from cis- to trans- Golgi) via bulk flow.
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