Calcium-binding and aggregation properties of parathyroid secretory protein-I (chromogranin A)

Secretory protein-I (SP-I, also referred to as chromogranin A) is a major acidic, sulfated glycoprotein of the parathyroid gland secretory granule. It is cosecreted with parathormone under calcium regulation. SP-I is similar if not identical to chromogranin A of the adrenal medulla chromaffin cell and has been found in most endocrine cells. Its function is unknown, but it may play a role in the packaging of secreted substances. We have examined the calcium-binding properties of purified bovine SP-I and find that it binds 14 or more Ca2+ at low affinity. Measurements of sedimentation equilibrium and light scattering reveal that in the absence of added Ca2+ SP-I is monomeric (molecular weight about 52,000), but as Ca2+ binding proceeds it aggregates. The adherence of the molecule to biological cell membranes is also enhanced by Ca2+. These properties are consonant with earlier speculation that SP-I serves as a 'condensing' agent within secretory granules.