We have investigated the formation of protein storage vacuoles in peas (Pisum sativum L.) in order to determine whether this organelle arises de novo during cotyledon development. A comparison of different stages in cotyledon development indicates that soluble protease activities decline and the amounts of storage proteins and the integral membrane protein of the protein body, alpha-TIP, increase during seed maturation. On linear sucrose density gradients we have been able to distinguish between two separate vesicle populations: one enriched in alpha-TIP, and one in TIP-Ma 27, a membrane protein characteristic of vegetative vacuoles. Both vesicle populations possess, however, PPase and V-ATPase activities. Conventionally fixed cotyledonary tissue at an intermediate stage in cotyledon development reveals the presence of a complex tubular-cisternal membrane system that seems to surround the pre-existing vacuoles. The latter gradually become compressed as a result of dilation of the former membrane system. This was confirmed immunocytochemically with the TIP-Ma 27 antiserum. Deposits of the storage proteins vicilin and legumin in the lumen, and the presence of alpha-TIP in the membranes of the expanding membrane system provide evidence of its identity as a precursor to the protein storage vacuole.
A Novel Membrane Protein That Is Transported to Protein Storage Vacuoles via Precursor-Accumulating Vesicles
