Redrawing compartmental boundaries in the exocytic pathway

Compartments can be defined both morphologically as well as biochemically. The former relies on a direct visualisation of membrane boundaries and the latter by the characterisation of enzymatic functions taking place within the compartment. The combination of morphology and biochemistry has led to the identification of several compartments within the exocytic pathway, each assumed to carry out independent functions in a logical succession [Palade, G. (1975) Science 189, 347-358]. However, recent findings show that resident enzymes are confined to not one but at least two adjacent compartments suggesting that morphological and biochemical definitions do not coincide. We will here discuss these findings and propose a model to explain how compartments are organised and maintained along the exocytic pathway.