Question

Discuss the potential fates of transmembrane proteins and explain where you would expect to find a particular domain of a plasma membrane transmembrane protein based on where this domain was found when the protein was still present at the ER membrane

Recall that some (but not most) proteins are brought to the ER lumen via posttranslational translocation. Discuss how this mechanism differs from cotranslational translocation.

Answer 1

Integral membrane proteins or transmembrane proteins located in the endoplasmic recticulum, Golgi, lysosomes and the plasma membrane, all are synthesized on the RER (Rough endoplasmic recticulum).

These transmembrane proteins are firstly inserted in the ER membrane instead of being released in the ER lumen. From the ER membrane, transport of integral membrane protein follows the same secretory protein pathway i.e from ER to Golgi, Golgi to plasma membrane or lysosome. During transport, the orientation of transmembrane protein is preserve i.e the end of the protein (N or C) that faces the lumen of ER will remain at the non cytosolic side after being inserted in to the plasma membrane and the other terminal (N or C) will remain at the cytosolic side after insertion into the plasma membrane.

Membrane proteins have four topologies:

Type 1: Single pass transmembrane protein having N terminal segment on the non cytosolic side and C terminal segment on the cytosolic side.

Type 2: Single pass transmembrane protein having C terminal segment on the non cytosolic side and N terminal segment on the cytosolic side.

Type 3: Single pass transmembrane protein having N terminal segment on the non cytosolic side and C terminal segment on the cytosolic side but it differs from type 1 by having single internal start transfer signal.

Type 4: Multipass transmembrane protein either having both N or C terminal on the same side i.e cytosolic / non cytosolic or having both C and N terminals on opposite sides.

Proteins synthesized by the membane bound ribosomes translocate into the ER membrane co-translationally. However some proteins are translocated after their synthesis has been completed (post translational translocation) and these proteins are synthesized by membrane free ribosomes i.e present in the cytosol.

During co-translational translocation, ribosomes provides the motive force that pushes the growing peptide into the ER lumen. In this, growing peptides having signal recognition sequence present at N-terminal is usually recognize by signal recognition particle (SRP) which is a ribonucleoprotein present in the cytosol. SRP binding stops the translation of nascent peptides. After interaction of SRP and its SRP receptor present on ER membrane , Nascent peptide enters through sec 61 translocation complex.

But during post translational translocation, additional proteins are needed for the unidirectional translocation of synthesized proteins i.e Sec62/Sec63 complex and the chaperone protein BiP. The substrate binding domain (SBD) of Bip helps in translocation of protein in the ER lumen.