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The topology of a protein in the endoplasmic reticulum (ER) refers to how the protein is oriented or positioned within the ER membrane. Proteins in the ER membrane can have different topologies depending on their function, structure, and the number of transmembrane segments they contain. Here are some common topologies of proteins in the ER:
Type I Single-Pass Transmembrane Protein:
These proteins have a single transmembrane domain (TM) that anchors them in the lipid bilayer.

The N-terminus is located in the cytoplasm, and the C-terminus is in the ER lumen. Signal sequence (N-terminus)
Cleaved Sequence Present
Stop Signal Present
Translocon stopped
Diffuse Laterally and Anchored in Lipid bilayer
Translation continues


Eg : LDL Receptor,Insulin Receptor
Type II Single-Pass Transmembrane Protein:
Similar to Type I, these proteins have a single TM domain.

(N) terminus is exposed on the cytosolic side.
C-terminus on the non-cytosolic side.
Single non-cleavable internal sequence.
The transmembrane sequence exits the translocon to anchor the protein in the lipid bilayer and the remainder of the polypeptide chain is translocated into the ER as translation proceeds.
Eg: Transferrin receptor, Golgi galactosyl transferase


However, the N-terminus is located in the ER lumen, and the C-terminus is in the cytoplasm.
TYPE III Single Pass Transmembrane
Similar to Type I
No cleavable signal sequence
Like Type II single cleavable internal signal sequence
Eg: Cyt P〰450
Multi-Pass Transmembrane Protein:
These proteins have multiple TM domains that traverse the lipid bilayer several times.
Spans the membrane multiple times
Its amino (N) terminus and C-terminus either on same side or different orientation
Even number Alpha helices- both N and C-terminus same side
Eg: CFTR, GLUT 1
Translation proceeds until a second transmembrane sequence is encountered. This causes the polypeptide chain to form a loop within the lumen of the ER; translation continues in the cytosol.