Aldosterone is our major hormone for regulating sodium ion concentration in the

Question

Aldosterone is our major hormone for regulating sodium ion concentration in the blood. It is secreted by the outer later of the adrenal cortex and acts on receptors in the distal portion of the kidney tubules. When aldosterone bind to receptors it increases the ability for reabsorption of sodium ions by increasing transcription of sodium ATPase pumps and sodium ion channels. The ATPase pumps are inserted into the apical membrane to increase entry of sodium from the tubules and the pumps are inserted into the basal membrane to pump sodium ions out into the interstitial fluid and ultimately the blood.

Explain the route through the organelles that the newly transcribed pump and channel mRNA will take from the nucleus to the insertion in the plasma membrane. Draw a diagram if you wish.  

Somehow the cell knows to transport the channel proteins to the apical surface and the pumps to the basal surface. How is this achieved?

Explanation / Answer

After gene expression in the nucleus, the mRNA exported into either cytosol or translated by the ribosomes on the RER (rough endoplasmic reticulum). It depends on the presence or absence of signal peptide coding sequence on mRNA. The proteins without signal peptide directly reach to the cytoplasm and then translated by the free ribosomes.

These proteins are later destined to other cell organelles like mitochondria, lysosomes and nucleus etc. most probably enzyme will chose this pathway. For example, these proteins are destined based on special sequences, if a protein is destined to the nucleus, so it must have NLS (nuclear localization signal).       

The proteins with signal peptide attach to the ribosomes on the RER and then translated and stored in the ER and then transferred to the Golgi complex. In these two organelles, proteins undergo several post-translational modifications like attachment of carbohydrates, hydroxylation, glycosylation, oligomerization and folding, etc. After completion of their post-translational modifications, they sort out into their destination based on their signal sequence.

In animals cells, proteins secreted into the plasma membrane (both apical and basal surfaces). For targeting to the plasma membrane, proteins have additional sequences (e.g., membrane spanning regions, stop transfer sequences, GPI anchors, etc.) which allow them to attach initially to the ER membrane.

As membrane material “flows” from the ER to the Golgi and finally to the plasma membrane, these proteins remain associated with the membrane and now appear on the plasma membrane surface.

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.