mu 1A-adaptin-deficient mice: lethality, loss of AP-1 binding and rerouting of mannose 6-phosphate receptors

The heterotetrameric AP-1 complex is involved in the formation of clathrin- coated vesicles at the trans-Golgi network (TGN) and interacts with sorting signals in the cytoplasmic tails of cargo molecules. Targeted disruption o f the mouse mu 1A-adaptin gene causes embryonic lethality at day 13.5. In c ells deficient in mu 1A-adaptin the remaining AP-1 adaptins do not bind to the TGN. Polarized epithelial cells are the only cells of mu 1A-adaptin-def icient embryos that show gamma-adaptin binding to membranes, indicating the formation of an epithelial specific AP-1B complex and demonstrating the ab sence of additional mu 1A homologs. Mannose 6-phosphate receptors are cargo molecules that exit the TGN via AP-l-clathrin-coated vesicles. The steady- state distribution of the mannose 6-phosphate receptors MPR46 and MPR300 in mu 1A-deficient cells is shifted to endosomes at the expense of the TGN, M PR46 fails to recycle back from the endosome to the TGN, indicating that AP -1 is required for retrograde endosome to TGN transport of the receptor.