The polarized epithelia‐specific μ1B‐adaptin complements μ1A‐deficiency in fibroblasts

The heterotetrameric AP‐1A adaptor complex of clathrin‐coated vesicles is ubiquitously expressed. The μ1‐adaptin subunit of the complex exists as the ubiquitous μ1A and the polarized epithelia‐specific μ1B, which are 80% identical. In polarized epithelia, μ1B 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 μ1B in μ1A‐deficient fibroblasts to test for μ1B 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 μ1A and μ1B 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.