3. how do multiple transmembrane proteins get inserted correctly into the membrane?

Multiple transmembrane proteins get inserted correctly into the membrane thanks to signaling sequences.

The amino acid chain of transmembrane proteins (pass through a membrane one or more times) are inserted into the membrane by translocation. Transmembrane proteins must enter the ER before their insertion in cytoplasm and their transport is co-translational (the process of translocation occurs while the protein is still being synthesized on the ribosome). Protein should contain:  

•Signal peptide or signal sequence is a sequence of amino acids at the N terminal that gets recognized by signal recognition particle (SRP). This sequence is involved in cotranslational translocation into ER, where the protein should be folded.

•A stop-transfer sequence-signals that translation of the protein should continue in the cytosol (not in ER).

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