Cisternal progression

Cisternal progression is one theory of protein transport through the Golgi. Instead of vesicles continually budding and fusing to static cisternae carrying cargo molecules, the cisternae themselves would move forward, towards the cell surface. Any resident of the Golgi not meant to travel to the cell surface or any other destination out of the Golgi, would be packaged in vesicles and moved back to the next cisternae. In this manner the cis (early most) cisternae would mature into the medial and the medial would mature into the trans cisternae by moving forward and receiving resident Golgi proteins from the cisternae before it. Both models of transport are considered to hold true (cisternal progression vs. bulk flow). Some cargo proteins may take one route while others may choose the alternative route. For instance, a typical vesicle is on the order of 60-90nm. If a large protein is, say 400nm in length, it would be too big to fit in the vesicle and therefore may be left packaged in the cisternae itself. This is basically where cisternal progression came from, the study if algal sclaes and collagen transport. This also brings up the idea that the Golgi may not be a static organelle. If the cisternae move forward and mature during transport, then what would happen if transport is halted? Do the cisternae disappear or do they just stop transporting and stay where they are? The answer is, the disappear. Actually the recycling of Golgi into the endoplasmic reticulum continues and the Golgi fuses with the ER, becoming no more. Those favoring cisternal progression use this fact to explain their hypothesis. For if the Golgi was a static organelle with vesicles budding and fusing from it, then it should be an oranelle regardless of transport, but this is not the case. Much controversy still continues with the idea of cisternal progression vs the bulk flow model.