Serglycin proteoglycan is sorted into zymogen granules of rat pancreatic acinar cells

Serglycin is known as a secretory granule proteoglycan in hematopoietic cells. In this study we identified a high-molecular-weight molecule in aggregated content proteins of zymogen granules of pancreatic acinar cells. The amino acid composition of the isolated protein showed high similarity to serglycin proteoglycan core protein. To confirm the expression of serglycin proteoglycan in pancreatic acinar cells we cloned the rat pancreas cDNA of serglycin core protein and detected the serglycin mRNA in pancreas tissue and AR4 - 2J cells by reverse transcription-PCR. In AR4 - 2J cells, transfected with serglycin fused to green fluorescent protein (EGFP), serglycin localized within a web-like pattern in the perinuclear space as well as with a punctate pattern distributed in the cytoplasm. The perinuclear structures colocalized with the Golgi membrane-associated protein p115 and the punctate structures with the secretory enzyme procarboxypeptidase A, indicating that the serglycin-EGFP fusion protein travels through compartments of the secretory pathway and is sorted into secretory granules. Using an antiserum against serglycin core protein immunofluorescence as well as immunogold electron microscopy analysis confirmed the subcellular distribution of serglycin proteoglycan in zymogen granules of pancreatic acinar cells. To prevent glycosylation of serglycin core protein we incubated AR4 - 2J cells with 2 mM p-nitrophenyl-beta-D-xylopyranoside (PNP-xyloside), which serves as alternate substrate for glycosaminoglycan chain attachment. Furthermore, we deleted the serine/glycine repeat region in the serglycin core protein. In both approaches the transfected serglycin-EGFP fusion protein could be detected predominantly in perinuclear Golgi membrane structures, while in control cells the serglycin fusion protein was mostly sorted into the secretory granules. Additionally, we show that sorting of secretory enzymes like amylase and procarboxypeptidase A into secretory granules was equally impaired when attachment of glycosaminoglycan chains to proteoglycan core proteins was inhibited.