Two types of secretory pathways are present in mammalian cells: constitutive secretion and regulated secretion (1). In the constitutive secretory pathway
which is found in all types of cells
secretory products are packed in small vesicles. Most of the proteoglycans and glycoproteins of the extracellular matrix are secreted in this way. The regulated secretory pathway
found in the more differentiated secretory cells
is mediated by specialized secretory granules. Hormones and neuropeptides are secreted in this way. Both the constitutive and regulated secretory pathways emanate from the trans-Golgi network (TGN).Recent studies have indicated that the granin family (secretogranins/chromogranins) plays an important role in the sorting and aggregation of secretory products in the trans-Golgi network (TGN)
and in the subsequent formation of secretory granules. The granin family is thought to be one of the mediators of the regulated secretory pathway (2-6). This family is widely expressed in endocrine (7-9) and neuronal cells (10-12)
where they are stored in secretory granules together with various peptide hormones and neuropeptides. Three acidic sulfated proteins
chromogranin A (Cg A)
secretogranin I (Sg I; also called chromogranin B; Cg B)
and secretogranin II (Sg II) are well known as the major proteins in the family. Recently
three acidic secretory proteins
1B1075 gene product (13)
HISL-19 antigen (14) and 7B2 (15)
have been thought to be members of the granin family
accordingly termed as Sg III
Sg IV and Sg V
respectively. However
it still remains open to discussion whether 1B1075 gene product
the HISL-19 antigen and 7B2 are true members of the granin family.In this review
the possible roles of granin family
especially Cg A
Sg I (Cg B) and Sg II in the regulated secretory pathway are discussed
focusing on the sorting
aggregation of proteins in the TGN
and subsequent secretory granule formation.
