The structural basis for the interaction of YXXØ-type sorting signals with μ1A has not been elucidated. However, X-ray crystallographic studies of the homologous μ2 subunit of AP-2 in complex with YXXØ-containing peptides revealed the presence of a binding site comprising two hydrophobic pockets for the Y and Ø residues (Owen and Evans, 1998). Notably, the residues that line the YXXØ-binding site in μ2, except for K420, are conserved in μ1A, suggesting that this protein has a similar binding site. Indeed, mutation of some of the conserved μ1A residues (i.e., F172, D174, AZD6244 W408, and R410) (Figure 2A) to alanine or serine abrogated interaction with the

cytosolic tail of TGN38 (Figure 2B), a TGN-localized, check details type I transmembrane protein having a prototypical YXXØ motif (YQRL, residues 350–353) (Ohno et al., 1995). Binding of the CAR tail to μ1A exhibited similar requirements (except for R410) (Figure 2B), indicating that the CAR YNQV signal binds to the conserved, canonical site. Surprisingly, binding of the TfR tail to μ1A was only abolished by mutation of W408 (Figure 2B). Thus, although the somatodendritic sorting signals in CAR and TfR both fit the YXXØ motif, the CAR signal binds to a canonical site, whereas the TfR signal binds to a different site that only shares a requirement for W408. The characterization

of the interactions shown in Figure 2B Astemizole allowed us to devise a dominant-negative approach to test for the involvement of μ1A in somatodendritic sorting. This approach consisted of overexpressing hemagglutinin (HA)-tagged μ1A-wild-type (WT) or μ1A-W408S constructs in hippocampal neurons and then examining the distribution

of TfR-GFP and CAR-GFP in these cells. Both μ1A proteins were equally incorporated into the endogenous AP-1 complex, as determined by immunoprecipitation with antibody to the HA epitope followed by immunoblotting with antibody to the γ-adaptin subunit of AP-1 (Figure 2C). Moreover, confocal fluorescence microscopy showed that both GFP-tagged μ1A-WT and μ1A-W408S colocalized with endogenous γ-adaptin and TGN38 to a juxtanuclear structure typical of the TGN/RE in the neuronal soma (Figures 2D and 2E), as well as to dendritic structures previously defined as “Golgi outposts” (Horton et al., 2005) (Figures 2D and 2E, diamonds). Colocalization was extensive, with Manders coefficients of ∼0.9. Overexpression of μ1A-WT had no effect on the somatodendritic localization of TfR-GFP and CAR-GFP, whereas overexpression of μ1A-W408S resulted in appearance of both receptors in the axon (Figures 3A and 3B; Figure S3E) (polarity indexes shown in Table 1 and Figure S3F). Overall axonal-dendritic polarization and the integrity of the axon initial segment (AIS) were not affected by μ1A-W408S overexpression (Figures S4A and S4C).