Munc13-4 interacts with syntaxin 7 and regulates late endosomal maturation, endosomal signaling, and TLR9-initiated cellular responses.

The molecular mechanisms that regulate late endosomal maturation and function are not completely elucidated, and direct evidence of a calcium sensor is lacking. Here we identify a novel mechanism of late endosomal maturation that involves a new molecular interaction between the tethering factor Munc13-4, syntaxin 7, and VAMP8. Munc13-4 binding to syntaxin 7 was significantly increased by calcium. Colocalization of Munc13-4 and syntaxin 7 at late endosomes was demonstrated by high-resolution and live-cell microscopy. Munc13-4-deficient cells show increased numbers of significantly enlarged late endosomes, a phenotype that was mimicked by the fusion inhibitor chloroquine in wild-type cells and rescued by expression of Munc13-4 but not by a syntaxin 7-binding-deficient mutant. Late endosomes from Munc13-4-KO neutrophils show decreased degradative capacity. Munc13-4-knockout neutrophils show impaired endosomal-initiated, TLR9-dependent signaling and deficient TLR9-specific CD11b up-regulation. Thus we present a novel mechanism of late endosomal maturation and propose that Munc13-4 regulates the late endocytic machinery and late endosomal-associated innate immune cellular functions.