ABSTRACT The autophagosomal SNARE STX17 (syntaxin 17) promotes lysosomal fusion and
degradation, but its autophagosomal recruitment is incompletely understood. Notably,
PtdIns4P is generated on autophagosomes and promotes fusion through an unknown mechanism.
Here we show that soluble recombinant STX17 is spontaneously recruited to negatively
charged liposomes and adding PtdIns4P to liposomes containing neutral lipids is sufficient
for its recruitment. Consistently, STX17 colocalizes with PtdIns4P-positive autophagosomes
in cells, and specific inhibition of PtdIns4P synthesis on autophagosomes prevents
its loading. Molecular dynamics simulations indicate that C-terminal positively charged
amino acids establish contact with membrane bilayers containing negatively charged
PtdIns4P. Accordingly, Ala substitution of Lys and Arg residues in the C terminus
of STX17 abolishes membrane binding and impairs its autophagosomal recruitment. Finally,
only wild type but not Ala substituted STX17 expression rescues the autophagosome-lysosome
fusion defect of STX17 loss-of-function cells. We thus identify a key step of autophagosome
maturation that promotes lysosomal fusion.
