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How does the endoplasmic reticulum help make and transport proteins?

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Artist's illustration of human nerve cells showing a central neuron with a green nucleus, branching dendrites, and glowing signals traveling along axons against a dark background. — cell Artist's conception of a human neuron.

The rough endoplasmic reticulum (RER) is a network of flattened sacs essential for the synthesis and export of proteins. Its role in this process is facilitated by ribosomes attached to its outer surface.12

Protein Synthesis

Protein production begins on a free ribosome, where a specific "signal sequence" at the start of the growing protein directs the ribosome to the RER membrane. A signal recognition particle (SRP) binds to the ribosome and the signal sequence, halting production until the complex docks with a receptor on the RER. Once attached, the protein threads through a channel called a translocon into the ER lumen. Inside, the signal sequence is typically removed by an enzyme, and the protein often undergoes glycosylation to become a glycoprotein before folding into its final shape.1

Transport

Once inside the lumen, proteins destined for secretion move to the ribosome-free transitional region of the ER. They are packaged into membrane-bounded transport vesicles that bud off the ER and travel through the cytoplasm to the Golgi complex for further processing and delivery.1