MATURATION OF PHAGOSOMES IS ACCOMPANIED BY CHANGES IN THEIR FUSION PROPERTIES AND SIZE-SELECTIVE ACQUISITION OF SOLUTE MATERIALS FROM ENDOSOMES

Maturation of phagosomes is characterized by changes in their polypept ides, phosphorylated proteins and phospholipid composition, Kinetic an alyses have shown that a variety of proteins associate and dissociate from latex-containing phagosomes at precise intervals during phagolyso some biogenesis. In an attempt to link these temporal biochemical modi fications to functional changes, we have examined the in vivo fusion p roperties of aging endosomes and phagosomes, Using an in vivo fusion a ssay at the electron microscope, we measured the rate of exchange of b ovine serum albumin-gold (5 and 16 nm particles) between endosomes and latex-bead-containing phagosomes. The results obtained indicate that the maturation of phagosomes is accompanied by changes of their fusion properties, Early phagosomes were shown to fuse preferentially with e arly endocytic organelles and to gradually acquire the ability to fuse with late endocytic organelles, Furthermore, the transfer of bovine s erum albumin-gold from endosomes to phagosomes is size-dependent, a pr ocess also modulated by the maturation of these organelles, in agreeme nt with the concept that transient fusion events occur between endosom es and phagosomes. Biochemical analysis showed variations in the level s of rab proteins associated with phagosomes during maturation while o ther 'fusion' proteins, including synaptobrevin1 and synaptobrevin2, r emained constant.