Getting specific about polarized sorting

In polarized epithelial cells, the clathrin adaptor complex AP-1B targets many proteins to the basolateral membrane, but how does AP-1B separate its cargos away from the ubiquitous AP-1A complex, from which it differs only slightly? A pair of papers in this issue identify several unique components of the AP-1B targeting system, and show that AP-1B segregates into a distinct membrane domain that may connect two basolateral sorting pathways.

Folsch et al. (page 351) show that, despite their strong homology, AP-1A and AP-1B complexes segregate onto distinct populations of clathrin-coated membranes in MDCK cells. In the Golgi apparatus, AP-1B specifically recruits components of the exocyst complex, which is thought to target transport vesicles to the basolateral plasma membrane. AP-1B also localizes to perinuclear regions that appear to be in a post-Golgi compartment that is...