Maturation of Golgi cisternae directly observed

Newly synthesized secretory proteins pass through the Golgi apparatus, which consists of multiple cisternae containing distinct populations of enzymes. Are the cargo proteins shuttled between cisternae in vesicles or do they remain in a cisterna while it is the Golgi enzymes that are removed and replaced? As predicted by the latter model – the cisternal maturation hypothesis – two groups have directly observed the replacement of one Golgi protein with another in individual cisternae, thus answering the question. However, its solution raises many more unknowns.