Compartmentation of proteins in the protein storage vacuole: A compound organelle in plant cells

Protein storage vacuoles (PSVs) in seed embryos store proteins during seed development and maturation. During germination these proteins are degraded to provide nutrients for the embryo. PSVs purified from mature seeds, however, contain a variety of hydrolytic enzymes that are capable of degrading the storage components; it has been unclear how both storage and lytic components could coexist in one package. Recent studies have provided new insights into the structural organization of PSVs that may help explain this seeming paradox. The PSV is a compound organelle, where a storage compartment and a lytic compartment are both present within the organelle, but separated by a membrane. Thus the PSV has within it a “Trojan horse”, which presumably is activated during germination to result in the destruction of the storage compartment. We postulate that a specialized multivesicular body pre-vacuolar organelle initially partitions storage and lytic functions, and that delivery of its components into the PSV could explain formation of the internal, membrane-bound lytic compartment.