Golgi-mediated transport of seed storage proteins

The great majority of seed proteins that are stored in the vacuole prior to desiccation are transported via the Golgi apparatus. In this organelle the y are separated from other products of the secretory pathway. Evidence is a ccumulating that the mechanism for segregation of storage proteins is diffe rent from that of soluble proteins destined for lytic vacuoles: it rarely s eems to require short targeting propeptides at the N- or C-terminus. Instea d, the three-dimensional conformation of the protein appears to be a critic al factor, leading to self-assembly into osmiophilic aggregates. Also unusu al is that this process starts immediately after entry into the Golgi appar atus, i.e. at the cis-cisternae, rather than at the trans-pole where acid h ydrolases are packaged into clathrin-coated vesicles. Storage protein aggre gates accumulate into so-called "dense" vesicles at the periphery of the ci sternae and are transported towards the trans-pole of the Golgi apparatus b y cisternal progression. Before the dense vesicles are released, clathrin-c oated vesicles form at their surface; however, the function of the latter r emains the object of speculation. In other eukaryotes, delivery of Golgi-de rived lumenal products to the vacuole does not occur directly, but via a pr e-vacuolar compartment. There is evidence that this is also the case for pl ants, and in developing pea cotyledons the pre-vacuolar compartment takes t he form of a large multivesicular body. Ultimately this appears to fuse in tote with the protein storage vacuole.