A 13-amino acid n-terminal domain of a basic proline-rich protein is necessary for storage in secretory granules and facilitates exit from the endoplasmic reticulum.

We have investigated the role of different domains of a salivary basic proline-rich protein in intracellular transport and sorting of proline-rich proteins to the secretory granules. We have cloned a full-length cDNA of a basic proline-rich protein from the rat parotid and expressed it in AtT-20 cells. It was correctly sorted into secretory granules as shown by EM immunolocalization and by its presence in 8-bromocyclic AMP-stimulated secretion. Deletion of the N-terminal thirteen amino acid domain upstream from the proline-rich domain eliminated storage whereas deletion of the C-terminal 20-amino acid domain downstream from the proline-rich domain had no effect. Intracellular transport of full-length and mutant proline-rich proteins was unusually slow due to slow exit from the endoplasmic reticulum. However, the rate of transport increased with increasing level of expression for the full-length protein and the C-terminal deletion mutant. In contrast, the rate of transport of the N-terminal deletion mutant was independent of the level of expression. These results imply that the N-terminal domain is necessary for both storage and efficient intracellular transport. Moreover, interactions (self-aggregation?) that mediate sorting may begin as early as the endoplasmic reticulum.