The lysosomal polypeptide transporter TAPL is stabilized by interaction with LAMP-1 and LAMP-2

TAPL (ABCB9) is a homodimeric polypeptide translocation machinery which transports cytosolic Peptides into the lumen of lysosomes for degradation. Since the function of proteins is strongly dependent on the interaction network involved, we investigated the interactome of TAPL. A proteomic approach allowed identification of the lysosome-associated membrane proteins LAMP-1 and LAMP-2B as the most abundant interaction partners. Albeit with low frequency, major histocompatibility complex II subunits were also detected. The interaction interface with LAMP was mapped to the four-transmembrane helices constituting the N-terminal domain of TAPL (TMD0). The LAMP proteins bind independently to TAPL. This interaction has influence on neither subcellular localization nor peptide transport activity. However, in LAMP-deficient cells, the half-life of TAPL is decreased by a factor of five, whereas another lysosomal membrane protein, LIMP-2, is not affected. Reduced stability of TAPL is caused by increased lysosomal degradation, indicating that LAMP proteins retain TAPL on the limiting membrane of endosomes and prevent its sorting to intraluminal vesicles.