The heterotetrameric AP-1A adaptor complex of clathrin-coated vesicles is ubiquitously expressed. The mu 1-adaptin subunit of the complex exists as the ubiquitous mu 1A and the polarized epithelia-specific mu 1B, which are 80% identical. In polarized epithelia, mu 1B is incorporated into the AP-1B complex, which is required for basolateral plasma membrane sorting of the low-density lipoprotein receptor. Binding of AP-1B to subdomains of the trans-Golgi network (TGN) appears to be part of the mechanism by which protein sorting is mediated. We expressed mu 1B in mu 1A-deficient fibroblasts to test for mu 1B function in non-polarized cells. AP-1B complexes were formed and bound to the TGN and to endosomes. Moreover, AP-1B restored the AP-1A-dependent sorting of mannose 6-phosphate receptors between endosomes and the TGN. This demonstrates that mu 1A and mu 1B do have overlapping sorting functions and indicates that AP-1A and AP-1B mediate protein sorting along parallel pathways between the TGN and endosomes in polarized epithelia.