The main sub cellular features of a protein secreting cell.

o   A protein secreting cell is metabolically very active

o   It receives signals via G protein coupled receptors.

o   It has a prominent nucleus which takes a large space of the cell. (Transcription, translation occurs in it.)

o   RER supports in synthesis of protein. So it is prominent in the cell. (RER has ribosome on its surface which involve in polypeptide synthesis using amino acids)

o   More mRNA, tRNA & rRNA make cytoplasm basophilic.

o   From RER synthesized polypeptides sent to Golgi apparatus for further modifications via vesicles. So large number of vesicles can be seen.

o   Well developed Golgi apparatus can be seen for post translational modifications of synthesized polypeptides.

o After that finished proteins exit from the cells by a secretory vesicles made out of Golgi apparatus, so large number of secretory vesicles can be seen.

How G protein help in signal transmission in to the cell from an incoming ligand & outline the steps.

• Ligand bind G protein coupled receptor which has α ,β & γ subunits.
• After binding, GTP in α dissociates from β & γ( all 3 subunits can produce physiological effects)
• Causes production of a 2^nd messenger eg:-cAMP via activation of adenylyl cyclase.
• cAMP produce activated protein kinase A( pK_A)  à  pK_A phosphorylate certain molecules & activate /inhibit  à brings about the physiological action
• When work is done , intrinsic GTPase activity of GTP-α converts GTP to GDP   à α ,β & γ subunits reassociates  à action terminates.