Together, these events prevent internalized

receptors from recycling to the plasma membrane and promote the subsequent delivery of ubiquitin-marked receptors to lysosomes. Ubiquitin-directed sorting has been extensively demonstrated in mammalian cells for the epidermal growth factor (EGF) receptor tyrosine kinase (Raiborg and Stenmark, 2009; Eden et al., 2012) and there is increasing evidence for ubiquitin-directed sorting of various 7TMRs, as reviewed elsewhere (Marchese et al., 2008; Shenoy, 2007; Kirkin and Dikic, 2007). In some cases, specific ubiquitin ligases and hydrolases controlling 7TMR endocytic trafficking have been identified, as previously reviewed elsewhere (Hislop and von Zastrow, 2011; Marchese et al., 2008; Shenoy, 2007), but the available information on this topic is presently limited to studies of 7TMR regulation in nonneural cell types. Some selleck neuromodulatory 7TMRs do not require ubiquitylation to undergo efficient endocytic delivery to lysosomes, and there is evidence for additional machinery directing this process. For example, internalized delta opioid receptors can

be effectively excluded from the recycling pathway and delivered to lysosomes even when their ubiquitylation is prevented by mutation of all cytoplasmic lysine residues (Tanowitz and Von Zastrow, 2002). Receptor ubiquitylation enhances but is not required for opioid receptor localization to intralumenal vesicles, and receptors can be delivered to lysosomes for inactivating proteolytic fragmentation even when transfer www.selleckchem.com/products/BKM-120.html to intralumenal vesicles is blocked (Hislop et al., 2009; Henry et al., 2011). Nevertheless, irrespective of whether or not ubiquitylation of receptors is allowed to

occur, the overall process of delta opioid receptor degradation requires the main components of the ESCRT machinery (Hislop et al., 2004). Accordingly, the present data suggest that discrete ubiquitin-independent and -dependent sorting mechanisms operate in series in the conserved ESCRT-dependent MVB pathway, with the ubiquitylation-independent MTMR9 mechanism operating effectively upstream and having the ability to effectively “force” internalized receptors to traffic to lysosomes even when their ubiquitylation is prevented (Henry et al., 2011). Evidence for ubiquitylation-independent sorting of internalized 7TMRs to lysosomes, as for the ubiquitin-directed sorting mechanism discussed above, is presently limited primarily to studies of nonneural cell types. The biochemical basis for ubiquitylation-independent lysosomal delivery of 7TMRs remains poorly understood in any system. One possibility is that internalized receptors are guided to lysosomes simply through “piggybacking” on a ubiquitin-directed cargo, as proposed for ubiquitin-independent trafficking in yeast (Macdonald et al.