The cargo in vacuolar storage protein transport vesicles is stratified

Developing pea seeds contain two functionally distinct vacuoles--lytic vacuoles and protein storage vacuoles (PSV). The Golgi apparatus of these cells has to discriminate between proteins destined for these vacuolar compartments. Whereas it is known that sorting into the lytic vacuole is performed via the conserved clathrin-coated vesicle pathway, sorting of proteins into the protein storage vacuole remains enigmatic. In developing pea cotyledons, the major storage proteins are sorted via 'dense vesicles'. In this report we examined the sorting of a minor protein of the protein storage vacuole, the sucrose-binding-protein homolog (SBP), along the secretory pathway employing immunoelectron microscopy on cryosectioned pea cotyledons. SBP follows the same vesicular route into the PSV as the main storage proteins legumin and vicilin, via the dense-vesicles. Furthermore, legumin and SBP are sorted together into the same dense vesicle population at the stack. Although soluble cargo proteins of the dense vesicles, they show a stratified distribution in the lumen of the dense vesicles. Whereas the legumin label is equally distributed across the lumen, the SBP label is concentrated at the membrane of the vesicle. This observation is discussed with respect to a putative receptor-mediated sorting of the proteins into the dense vesicles.