Which lysosomal membrane protein is required for fusion with phagosomes?
Phagosome maturation in macrophages and neutrophils converts a nascent bacteria-containing vacuole into a microbicidal phagolysosome through sequential interactions with early endosomes, late endosomes and terminal lysosomes. Acquisition of lysosomal membrane proteins marks progression, with lysosome-associated membrane proteins 1 and 2, LAMP1 and LAMP2, being the most abundant heavily glycosylated proteins delivered to maturing phagosomes, protecting membrane from hydrolases. Studies using LAMP-1 and LAMP-2 knockout mice and combined siRNA show single loss causes mild delay while double deficiency impairs recruitment of Rab7 GTPase, loss of early marker EEA1 and failure of lysosomes to fuse, indicating cooperative requirement for tethering and SNARE complex assembly alongside syntaxin 7 and VAMP8. Additionally, phagosome resolution into lysosome-derived vesicles depends on clathrin coats, actin polymerization via Arp2/3 and microtubule motors that fragment the vacuole after cargo degradation, recycling membrane for reuse. Clathrin adaptors AP1 and EpsinR also deliver lysosomal phosphatases via Rab5 early endosomes to expand phagocytic cup. Thus both structural LAMP proteins and clathrin-mediated membrane remodeling cooperate sequentially in phagolysosome formation and turnover for host defense.
Ref: Huynh et al., EMBO Journal 2007: LAMP Proteins Are Required for Fusion of Lysosomes with Phagosomes.