Toward a Biochemical Definition of the Endosomal Compartment
The eucaryotic cell is highly compartmentalized, containing many functionally and structurally distinct membrane-bound organelles. Subcellular fractionation is a powerful technique being applied to the purification of individual organelles so that they might be studied in biochemical detail. A complete biochemical definition of organelles of the vacuolar system involved in membrane transport along either the endocytic or exocytic pathways requires their definition in both static and dynamic terms. An organelle is defined in static terms by its own specific set of resident proteins, which give it its structural and functional identity. An organelle is defined in dynamic terms by its temporal location along an intracellular membrane traffic route based on the kinetics of protein transit into and through each organelle along either the endocytic or exocytic pathway. To completely define an organelle in static terms would require its purification to homogeneity and subsequent biochemical characterization. To adequately define an organelle in dynamic terms does not necessarily require that an organelle be purified to homogeneity but instead requires a rapid, high-yield procedure that clearly resolves the organelle from other organelles along a given pathway. This review discusses the novel method of free flow electrophoresis that we have applied to both the preparation of highly purified endosomal fractions for their biochemical characterization and to the analytical characterization of the kinetics of transit of endocytic tracers through the organelles of the endocytic pathway.
KeywordsEarly Endosome Late Endosome Endocytic Pathway Endosomal Compartment Dynamic Term
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