Heterogeneity of proteins synthesized, stored, and released by the bag cells of Aplysia californica

The biosynthesis, storage, and release of specific proteins in the bag cells of Aplysia californica were studied. In the soma, a 25000 dalton precursor molecule is synthesized, and is processed to a 6000 dalton protein, which in turn is converted into a≤3000 dalton protein (Fig. 1). Pulse-chase experiments show that the lower molecular weight products disappear from the cell body after 20 hrs of chase (Fig. 1 B), but that this disappearance can be blocked by colchicine (Fig. 1 C), an inhibitor of axoplasmic flow. The primary stored form of low molecular weight proteins in the bag cell soma is 6000 daltons with a minor peak at ≤3000 daltons (Fig. 2A), and it is these protein classes which disappear with release of hormones from the bag cells (Fig. 2B and C). The proteins which are released from the neurohemal area by potassium depolarization are heterogeneous, and are composed of 2 major peaks at 6000 and ≤3000 daltons, and 1 minor peak at 12000 daltons (Fig. 3B). The release of these proteins was dependent upon the presence of calcium ion. A hypothetical model of the intracellular mechanisms interposed between biosynthesis and release of the proteins is presented (Fig. 4), and the biological significance of the data is discussed.