Mediatory molecules that fuse autophagosomes and lysosomes.

Autophagy functions to degrade intracellular foreign microbial invaders by a process that is termed xenophagy (antimicrobial autophagy). Xenophagosomes undergo a stepwise maturation process culminating in a fusion event with lysosomes, after which the cargoes are degraded. Recent investigations by our laboratory demonstrate that endocytic soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) are involved in the fusion between xenophagosomes and lysosomes. Knockdown of the combinational SNARE proteins Vti1b and VAMP8 with siRNAs disturbs the colocalization of LC3 with LAMP-1. We also find that the invasive efficiency of group A Streptococcus into cells is not altered by knockdown of VAMP8 or Vti1b, whereas cellular bactericidal efficiency is significantly diminished, indicating that xenophagy is functionally impaired. In addition, knockdown of these SNAREs inhibits the fusion of canonical autophagosomes with lysosomes. Together, these findings indicate that VAMP8 and Vti1b mediate fusion with lysosomes in both antimicrobial and canonical autophagy.