The timing of PI(3)P accumulation (white) on phagosomes varies.
Using ratiometric fluorescence microscopy, the authors followed the maturation of individual macrophage phagosomes containing IgG-opsonized erythrocytes. Most of the results were uniform and unsurprising. Actin is present briefly during phagosome formation, and then all of the phagosomes sequentially associate with the small GTPases Rab5a and Rab7. The lysosomal membrane protein LAMP-1 associates with the phagosomes after Rab7, consistent with lysosomal fusion.
A marker for phosphatidylinositol 3-phosphate (PI[3]P), however, reveals two distinct populations of erythrocyte-carrying phagosomes. In one population, PI(3)P levels spike rapidly after phagosome formation, and then fall to undetectable levels within 20 min. In the other population, PI(3)P rises slowly...
