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Abstract
Multivesicular bodies (MVBs) are endosomes or prevacuolar compartments. The lumens
of their internal vesicles are thought to be topologically equivalent to cytoplasm
and their membranes direct proteins and lipids for degradation. Here, we describe
a new MVB function; in certain plant MVBs, the internal vesicles contain lytic enzymes
and the surrounding 'soup' is a storage compartment. Separate vesicular pathways deliver
proteins to the storage and lytic compartments. Recent data indicate that mammalian
secretory lysosomes also have two compartments served by separate vesicular pathways.
The formation of separate storage and lytic compartments within MVBs poses problems
for membrane organization and topology that have not previously been considered in
the literature. We offer a hypothetical model to address these problems.