Proteins are synthesized inside secretory vesicles by ribosomes (R). Secretory vesicles mature and are stored until a secretory stimulus is received.Proteins are synthesized inside secretory vesicles by ribosomes (R). Secretory vesicles mature and are stored until a secretory stimulus is received.
Macromolecules cannot cross the plasma membrane. At first sight, this might seem to be an insurmountable problem for a protein secreting cell but the secret to protein secretion is to synthesize proteins for export within endosomes. Topologically at least, these proteins are never inside the cell and so do not have to cross the cell membrane to get out. Proteins are secreted when the containing vesicle fuses with the plasma membrane in the process of exocytosis.
Synthesis of secretory proteins begins with gene transcription and manufacture of messenger RNA to carry the sequence information from the nucleus to ribosomes in the cytoplasm. Secretory proteins start with a 'signal sequence' which targets the developing polypeptide to the ER. where it is N-glycosylated and folded into the correct three-dimensional structure. Small membrane vesicles carry proteins from the ER through several layers of the golgi apparatus for additional processing and 'packaging' for export.
Proteins move by default onwards from the ER; those destined to remain in the cell contain specific 'retention sequences' to segregate them from secretory proteins. Secretory proteins are concentrated within golgi condensing-vacuoles and stored in secretory vesicles. As these mature they are transported close to the apical membrane.
In response to a secretory stimulus, secretory vesicles fuse with the plasma membrane and discharge their contents outside the cell.
The secretory process may be divided into four stages. Synthesis, segregation & packaging, storage and release. Each of these stages is regulated by phosphorylation of target proteins by cAMP dependent pKA.
Therefore an increase in cAMP stimulates
Thus, an increase in the level of cAMP within the cell will stimulate every step involved in protein secretion..
cAMP binding to the regulatory subunit of pKA releases and activates the catalytic subunit. The catalytic subunit phosphorylates and upregulates many components of the secretory pathway including exocytosis
The pivotal role of cAMP in regulation of exocytosis in salivary cells is a bit weird. In just about every other secretory cell type, exocytosis is regulated by calcium.