Receptor binding peptide ligands

The retrieval mechanism for the M6P receptor resembles the one previously described for the H/KDEL receptor [53]. Optimal binding of M6P receptor to M6P occurs at pH 6.5-6.7, the pH found in the TGN. When transport vesicles arrive at late endosomes, the pH is lowered by the action of H+ pumps. The affinity of the M6P receptor for its ligands is reduced at acid pHs, resulting in M6P receptor releasing the M6P in the late endosome. As a result, transport of lysosomal hydrolases occurs unidirectionally. Once the M6P receptor releases M6P-bearing hydrolases, the receptor can be returned to the TGN for reuse. Transport of the M6P receptor to either TGN or late endosome relies on signal peptides on the cytoplasmic tail region of the M6P receptor. 

FIGURE 18-10 Serpentine (seven-transmembrane-domain) receptors for peptides have binding for their peptide ligands within the membrane... 

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