It is not clear how protein cargo is sorted to and retained in forming regulated secretory granules (RSG). Here, the sulfated mucin-type glycoprotein pro-Muclin was tested for its ability to induce RSG in the poorly-differentiated rat pancreatic cell line AR42J. AR42J cells express RSG content proteins but they fail to make granules. Adenovirus-pro-Muclin infected AR42J cells store amylase, accumulate RSG, and respond to hormonal stimulation by secreting the stored protein. Expression of pro-Muclin combined with the inducing effect of dexamethasone resulted in a significant enhancement of the efficiency of regulated secretion. The effect of pro-Muclin was a strong decrease in constitutive secretion compared to dexamethasone-induction alone. A pro-Muclin construct missing the cytosolic tail domain was less effective at improving the efficiency of regulated secretion compared to the full-length construct. Increased expression of cargo (using adenovirus-amylase) also modestly enhanced regulated secretion, indicating that part of pro-Muclin's effect may be due to increased expression of cargo protein. Overall, The data show that pro-Muclin acts as a sorting receptor that can induce RSG, and that its cytosolic tail is important in this process.