Specificity of binding of clathrin adaptors to signals on the mannose‐6‐phosphate/insulin‐like growth factor II receptor.

Adaptors mediate the interaction of clathrin with select groups of receptors. Two distinct types of adaptors, the HA‐II adaptors (found in plasma membrane coated pits) and the HA‐I adaptors (localized to Golgi coated pits) bind to the cytoplasmic portion of the 270 kd mannose 6‐phosphate (M6P) receptor‐a receptor which is concentrated in coated pits on both the plasma membrane and in the trans‐Golgi network. Neither type of adaptor appears to compete with the other for binding, suggesting that each type recognizes a distinct site on the M6P receptor tail. Mutation of the two tyrosines in the tail essentially eliminates the interaction with the HA‐II plasma membrane adaptor, which recognizes a ‘tyrosine’ signal on other endocytosed receptors (for example, the LDL receptor and the poly Ig receptor). In contrast, the wild type and the mutant M6P receptor tail (lacking tyrosines) are equally effective at binding HA‐I adaptors. This suggests that there is an HA‐I recognition signal in another region of the M6P receptor tail, C‐terminal to the tyrosine residues, which remains intact in the mutant. This signal is presumably responsible for the concentration of the M6P receptor, with bound lysosomal enzymes, into coated pits which bud from the trans‐Golgi network, thus mediating efficient transfer of these enzymes to lysosomes.