The phaseolin vacuolar sorting signal promotes transient, strong membrane association and aggregation of the bean storage protein in transgenic tobacco

Vacuolar storage proteins of the 7S class are co-translationally introduced into the endoplasmic reticulum and reach storage vacuoles via the Golgi complex and dense vesicles. The signal for vacuolar sorting of one of these proteins, phaseolin of Phaseolus vulgaris, consists of a four-amino acid hydrophobic propeptide at the C-terminus. When this sequence is deleted, phaseolin is secreted instead of being sorted to vacuoles. It is shown here that in transgenic tobacco plants newly-synthesized phaseolin has unusual affinity to membranes and forms SDS-resistant aggregates, but mutated phaseolin polypeptides that are either secreted or defective in assembly do not have these characteristics. Association to membranes and aggregation are transient events: phaseolin accumulated in vacuoles is soluble in the absence of detergents and is not aggregated. Association to membranes starts before the phaseolin glycan acquires a complex structure and therefore before the protein reaches the medial or trans-cisternae of the Golgi complex. These results support the hypothesis of a relationship between aggregation and vacuolar sorting of phaseolin and indicate that sorting may start in early compartments of the secretory pathway.