Cellular and Tissue Biology: Protein Localization and Membrane Insertion

Moving to Cellular and Tissue Level

• Transition from molecular level (genes, genetics) to cellular and tissue level.
• Tissues are communities of cells.
• Focus on how proteins form structures and how they are localized within the cell.

Protein Localization

• Key question: How do proteins go to the right position in the cell?
• Receptors (e.g., EGF receptor) are membrane proteins located on the cell surface.

Cell Membrane Structure

• Cell membrane is a lipid bilayer.
• Structure:
  • Hydrophilic heads face the aqueous environment (cytoplasm and outside of the cell).
  • Hydrophobic tails are in the middle of the membrane.
• Hydrophobic interactions keep the membrane stable.

Inserting Receptors into the Membrane

• Problem: How to get the receptor into the membrane after it's made by the ribosome?
• Energetic barrier: Forcing the hydrophobic receptor region through the hydrophobic membrane is difficult.
• Solution: Signal sequence and Signal Recognition Particle (SRP).

Signal Sequence

• The protein comes out of the ribosome, assembling amino acids one by one.
• At the front (Nh2 group) of the protein, there is a sequence of amino acids called the signal sequence.
• The signal sequence is present if the protein is going to the membrane or secreted from the cell.
• Different signal sequences for different destinations (e.g., nuclear localization signal for the nucleus, signal sequence for the mitochondria).

Signal Recognition Particle (SRP)

• SRP recognizes the signal sequence.
• SRP binds to the signal sequence.
• SRP receptor sits on the endoplasmic reticulum (ER).
• SRP brings the newly synthesized protein to the surface of the ER.
• The ribosome continues making the protein.

Translocation Channel

• The SRP receptor translates the protein into a transmembrane protein, kind of like a tunnel.
• The protein is threaded through the membrane of the endoplasmic reticulum via a hydrophilic channel.
• The center of this channel is aqueous, and its called the translocation channel.

Rough Endoplasmic Reticulum

• Ribosomes on the surface of the ER are pushing their protein into the inside of the endoplasmic reticulum.
• This is called rough endoplasmic reticulum.
• Indicates that the cell is making membrane proteins or secreted proteins.

Hydrophobic Stop Sequence

• The translocation channel allows hydrophilic proteins to pass through the hydrophobic membrane.
• When a hydrophobic region of the protein reaches the channel, it gets stuck and exits into the membrane.
• This is called a hydrophobic stop sequence because it stops the threading of the protein.

Signal Sequence Removal

• Another protein comes by and cuts off the signal sequence.

Final Transmembrane Protein

• The protein ends up spanning the ER membrane with a front (Nh2 head) and a back (COOH).
• The protein has been synthesized and inserted into the membrane.

Next Steps

• The protein sitting across the membrane of the ER is not at the surface of the cell yet.
• Next lecture: How the protein gets from the ER to the surface of the cell.