Apical targeting in polarized epithelial cells: there's more afloat than rafts

Most metazoan cells are 'polarized'. A crucial aspect of this polarization is that the plasma membrane is divided into two or more domains with different protein and lipid compositions - for example, the apical and basolateral domains of epithelial cells or the axonal and somatodendritic domains of neurons. This polarity is established and maintained by highly specific vesicular membrane transport in the biosynthetic, endocytic and transcytotic pathways. Two important concepts, the 'SNARE' and the 'raft' hypotheses, have been developed that together promise at least a partial understanding of the underlying general mechanisms that ensure the necessary specificity of these pathways.