The role played by subunit assembly in the intracellular transport of the bean storage protein phaseolin, a soluble trimeric glycoprotein, was investigated using Xenopus oocytes injected with RNA. We show that phaseolin assembly is dependent upon the level of synthesis of the protein and is required for intracellular transport out of the endoplasmic reticulum. We also show that a fraction of the assembled phaseolin is permanently retained in a post-endoplasmic reticulum compartment. Deletion of the C-terminal alpha-helical domain fully prevents in vivo assembly but not endoplasmic reticulum retention. This indicates that this domain is necessary for trimerization but not for interactions of unassembled subunits with endoplasmic reticulum components. The truncated phaseolin has high in vivo stability. The potential implications of these findings on the possibility to improve the nutritional value of phaseolin through genetic engineering are discussed.