Previous studies had established that interactions of tyrosine-based signals with the μ subunits of AP-2, AP-3, and AP-4 mediate various cargo sorting events, including rapid internalization from the plasma membrane, transport to lysosomes and melanosomes, and direct delivery from the TGN to endosomes (Bonifacino and Traub, 2003; Robinson, 2004;

Burgos DNA Damage inhibitor et al., 2010). The μ1A subunit of AP-1 was also known to interact with YXXØ-type signals (Ohno et al., 1995), but the functional significance of these interactions remained unclear. Our findings now show that YXXØ-μ1A interactions play a critical role in cargo sorting to the neuronal somatodendritic domain. The YNQV sequence from CAR behaves as a typical YXXØ signal, in that both the Y and V residues are required for somatodendritic sorting as well as interaction with μ1A (Figure S3) (Carvajal-Gonzalez

et al., 2012). Furthermore, this sequence binds to a site on μ1A that is similar to the structurally defined YXXØ-binding site on μ2 (Figure 2) (Owen and Evans, 1998). The YTRF sequence from TfR also fits the canonical YXXØ motif, and both the Y and F residues are necessary for somatodendritic sorting (Figure 1) and μ1A binding (Figure S1). However, this sequence seems to bind to a different site on μ1A that only shares W408 with the conserved Epigenetic inhibitor library site ADAMTS5 (Figure 2). This observation points to a potentially

new mode of signal recognition by μ subunits. Our findings highlight both similarities and differences in the mechanisms of somatodendritic sorting in neurons and basolateral sorting in epithelial cells. Among the similarities, interactions of signals with the μ1 subunit of AP-1 underlie both of these polarized sorting events. In addition, the same YXXØ signal in CAR, YNQV, mediates somatodendritic (Figure S3) and basolateral sorting (Cohen et al., 2001; Carvajal-Gonzalez et al., 2012). In the case of TfR, however, basolateral sorting does not depend on the YXXØ signal, YTRF, but on a noncanonical sequence, GDNS (residues 31–34) (Odorizzi and Trowbridge, 1997). We found that mutation of the GDNS sequence has no effect on somatodendritic sorting of TfR (data not shown), in agreement with results from a previous deletion analysis (West et al., 1997). Another key difference is that basolateral sorting of various cargoes, including TfR and CAR, depends mainly on the epithelial-specific μ1B instead of the ubiquitous μ1A (Fölsch et al., 1999; Gravotta et al., 2012; Carvajal-Gonzalez et al., 2012). These variations probably represent adaptations of a basic molecular recognition event to the need for achieving polarized sorting in cell types with very different structural and functional organizations.