Sorting of the vasopressin prohormone into the regulated secretory pathway

The sorting of soluble proteins into the regulated secretory pathway (RSP) involves aggregation, but whether an additional sorting domain is also required is not clear. By fusing vasopressin prohormone (proVP) fragments to green fluorescent protein (eGFP) we have determined whether a sorting domain can function independently of the aggregative neurophysin domain. Although eGFP itself can be immunolocalised in the RSP of Neuro2A and AtT20 cells, most of the protein enters the constitutive pathway, and is found in the culture medium. In contrast, the N-terminal 27 residues of proVP promote residence in the RSP. Furthermore, only the processed form of this fusion was secreted when stimulated. We suggest a sorting mechanism based on the recognition of a sorting motif, the efficiency of which is enhanced by neurophysin aggregation.