Direct delivery of procathepsin D to phagosomes: implications for phagosome biogenesis and parasitism by Mycobacterium

Phagosome maturation is characterized by the sequential acquisition and loss of proteins by the phagocytic vacuole during the formation of an acidic and hydrolytic compartment where degradation of the phagocytosed particle occurs. Transfer of proteins to the maturing phagosome occurs by fusion with a range of vesicles. Here we describe direct fusion of early phagosomes with vesicles that appear to be derived from the biosynthetic pathway. In mouse bone marrow macrophages, the 51 kDa proform of cathepsin D was found in vesicles of the ER/Golgi network that could be discriminated from endosomal vesicles which in turn contained the 46 and 30 kDa processed forms of the enzyme. Procathepsin D was acquired by phagosomes formed around inert particles such as IgG-coated beads and could be "protected" by blocking acidification with Bafilomycin A1. Mycobacterium avium-containing vacuoles from established infections possessed both pro- and processed cathepsin D similar to early bead-containing phagosomes. In contrast phagosomes harboring dead mycobacteria demonstrated markedly enhanced acquisition of the 46kDa form within 4 h post internalization and only low levels of procathepsin D.