Identification of a basolateral sorting signal for the m(3) muscarinic acetylcholine receptor in Madin-Darby canine kidney cells

Muscarinic acetylcholine receptors (mAChRs) can be differentially localized in polarized cells. To identify potential sorting signals that mediate mAC hR targeting, we examined the sorting of mAChRs in Madin-Darby canine kidne y cells, a widely used model system. Expression of FLAG-tagged mAChRs in po larized Madin-Darby canine kidney cells demonstrated that the M-2 subtype i s sorted apically, whereas M-3 is targeted basolaterally, Expression of M-2 /M-3 receptor chimeras revealed that a al-residue sequence, Ser(271)-Ser(29 1), from the M-3 third intracellular loop contains a basolateral sorting si gnal. Substitution of sequences containing the M-3 sorting signal into the homologous regions of M-2 was sufficient to confer basolateral localization to this apical receptor. Sequences containing the M-3 sorting signal also conferred basolateral targeting to M-2 when added to either the third intra cellular loop or the C-terminal cytoplasmic tail. Furthermore, addition of a sequence containing the M-3 basolateral sorting signal to the cytoplasmic tail of the interleukin-2 receptor alpha -chain caused significant basolat eral targeting of this heterologous apical protein. The results indicate th at the M-3 basolateral sorting signal is dominant over apical signals in M- 2 and acts in a position-independent manner. The M-3 sorting signal represe nts a novel basolateral targeting motif for G protein-coupled receptors.