Immunogold localisation of ubiquitin-protein conjugates in Sf9 insect cells. Implications for the biogenesis of lysosome-related organelles.

Immunogold electron microscopy with antibodies which primarily detect ubiquitin-protein conjugates shows conjugate-specific gold particles enriched severalfold in acid phosphatase-positive lysosomes and multivesicular bodies in insect Sf9 cells. The observations demonstrate that ubiquitinated proteins are associated with small acid phosphatase-containing primary lysosomes (transport vesicles) and indicate a pathway in which primary lysosomes fuse with multivesicular bodies to generate mature lysosome-related structures.