, 1999) and neurons (Yudowski et al., 2006; Yu et al., 2010), and the sorting activity of this sequence does not require cytoplasmic lysine residues that represent potential sites of receptor ubiquitylation (Hanyaloglu and von Zastrow, 2007). A variety of such “recycling sequences” have been identified in other 7TMRs, but not all are PDZ motifs. An interesting example is the mu opioid

receptor, whose recycling is promoted by a discrete, PDZ-independent C-terminal sequence that is devoid of lysine residues and critically depends on two leucine residues separated by two other amino acids (L-x-x-L) (Yu et al., 2010; Tanowitz and von Zastrow, 2003). This system INCB024360 concentration of endocytic fate determination confers additional regulation and KU-57788 concentration diversity of

7TMR regulation. For example, phosphorylation of the PDZ motif present in the beta-adrenergic receptor tail blocks its recycling activity and results in flexible rerouting of internalized beta-adrenergic receptors to the lysosomal downregulation pathway (Cao et al., 1999). Alternative splicing of mu opioid receptor transcripts creates variant receptors that lack the “L-x-x-L” recycling sequence and thus preferentially downregulate rather than recycle after endocytosis (Tanowitz et al., 2008). Both PDZ-dependent sequences derived from beta-adrenergic receptors and the discrete PDZ-independent sequence derived from mu opioid receptors have been explicitly shown to promote efficient sorting of internalized 7TMRs into the recycling pathway in neurons (Yu et al., 2010). The biochemical machinery that mediates Casein kinase 1 sequence-directed recycling has only

recently begun to come into focus, based largely on study of PDZ motif-directed recycling of beta-adrenergic receptors (Figure 2C). The critical trans-acting protein recognizing the recycling sequence present in the adrenergic receptor cytoplasmic tail is sorting nexin 27 (SNX27) ( Lauffer et al., 2010). Sorting nexins comprise a diverse family of cytoplasmic proteins that share a phosphoinositide-binding “SNX-PX” domain linking them to endosome and/or plasma membranes, and members of the sorting nexin family are found in diverse organisms ( Worby and Dixon, 2002; Carlton et al., 2005). SNX27, an early endosome-associating sorting nexin that is the only known family member to possess a PDZ domain, is restricted to metazoans. Depleting SNX27 inhibits recycling of both the beta 1 and beta 2 adrenergic receptors and increases receptor delivery to lysosomes, effectively phenocopying mutation of the respective C-terminal recycling sequences. SNX27 is highly expressed in neurons and its expression is subject to robust regulation by psychostimulant drugs ( Kajii et al., 2003). Accordingly, mechanistic elucidation of the sequence-directed recycling machinery suggests the existence of still more flexibility in the control of neuromodulatory 7TMR trafficking in vivo.