Lysosomal acid phosphatase (LAP) is synthesized as a type I membrane glycoprotein and targeted to lysosomes via the plasma membrane. Its cytoplasmic tail harbours a tyrosine‐containing signal for rapid internalization. Expression in Madine‐Darby canine kidney cells results in direct sorting to the basolateral cell surface, rapid endocytosis and delivery to lysosomes. In contrast, a deletion mutant lacking the cytoplasmic tail is delivered to the apical plasma membrane where it accumulates before it is slowly internalized. A chimeric protein, in which the cytoplasmic tail of LAP is fused to the extracytoplasmic and transmembrane domain of the apically sorted haemagglutinin, is sorted to the basolateral plasma membrane. A series of truncation and substitution mutants in the cytoplasmic tail was constructed and comparison of their polarized sorting and internalization revealed that the determinants for basolateral sorting and rapid internalization reside in the same segment of the cytoplasmic tail. The cytoplasmic factors decoding these signals, however, tolerate distinct mutations indicating that different receptors are involved in sorting at the trans‐Golgi network and at the plasma membrane.

Lysosomal acid phosphatase (LAP) is rapidly internalized from the cell surface due to a tyrosine‐containing internalization signal in its 19 amino acid cytoplasmic tail. Measuring the internalization of a series of LAP cytoplasmic tail truncation and substitution mutants revealed that the N‐terminal 12 amino acids of the cytoplasmic tail are sufficient for rapid endocytosis and that the hexapeptide 411‐PGYRHV‐416 is the tyrosine‐containing internalization signal. Truncation and substitution mutants of amino acid residues following Val416 can prevent internalization even though these residues do not belong to the internalization signal. It was shown recently that part of the LAP cytoplasmic tail peptide corresponding to 410‐PPGY‐413 forms a well‐ordered beta turn structure in solution. Two‐dimensional NMR spectroscopy of two modified LAP tail peptides, in which the single tyrosine was substituted either by phenylalanine or by alanine, revealed that the tendency to form a beta turn is reduced by 25% in the phenylalanine‐containing peptide and by approximately 50% in the alanine‐containing mutant peptide. Our results suggest, that in the short cytoplasmic tail of LAP tyrosine is required for stabilization of the right turn and that the aromatic ring system of the tyrosine residue is a contact point to the putative cytoplasmic receptor.

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