Phagosomes Fuse with Late Endosomes and/or Lysosomes by Extension of Membrane Protrusions along Microtubules: Role of Rab7 and RILP

TLDR

Nascent phagosomes must undergo fusion and fission reactions to acquire microbicidal properties essential for innate immunity. This study demonstrates that phagosome maturation involves the GTPase Rab7. Rab7 on phagosomes recruits the effector RILP, which links them to the dynein‑dynactin motor complex, driving tubule extension and fusion with late endosomes/lysosomes, as shown by fluorescence and electron microscopy. Rab7 recruitment precedes and is essential for phagosome fusion with late endosomes/lysosomes; dynein‑driven tubule extension is required, as truncation of RILP’s motor‑binding domain blocks this process, proving that maturation depends on Rab7‑mediated RILP recruitment and microtubule‑associated motor activity.

Abstract

Nascent phagosomes must undergo a series of fusion and fission reactions to acquire the microbicidal properties required for the innate immune response. Here we demonstrate that this maturation process involves the GTPase Rab7. Rab7 recruitment to phagosomes was found to precede and to be essential for their fusion with late endosomes and/or lysosomes. Active Rab7 on the phagosomal membrane associates with the effector protein RILP (Rab7-interacting lysosomal protein), which in turn bridges phagosomes with dynein-dynactin, a microtubule-associated motor complex. The motors not only displace phagosomes in the centripetal direction but, strikingly, promote the extension of phagosomal tubules toward late endocytic compartments. Fusion of tubules with these organelles was documented by fluorescence and electron microscopy. Tubule extension and fusion with late endosomes and/or lysosomes were prevented by expression of a truncated form of RILP lacking the dynein-dynactin-recruiting domain. We conclude that full maturation of phagosomes requires the retrograde emission of tubular extensions, which are generated by activation of Rab7, recruitment of RILP, and consequent association of phagosomes with microtubule-associated motors.