Shweta Patro | Exam # 2: Take Home Problem – GPIb | Due (2 nd Revision): 11/11/09 1. The ER signal sequence is generally 16 to 30 amino acids long. The ER signal sequence is also found at the N terminus of the protein, meaning it is found at the beginning of the polypeptide chain. The residues from position 1 to position 25 (mgsgprgalsllllllappsrpaag) fit these criteria. 2. After the polypeptide is made by the ribosome it can be released into different parts of the cell (depending on its function). Proteins that function in the cytosol (Class I) are released and are further folded or processed in the cytosol. But Class II - Class IV proteins (transmembrane proteins like GPIb are Class III) are directed along with their ribosomes to the rough ER membrane. This direction is done by the ER signal sequence. Once the signal sequence is translated into the signal peptide, the signal recognition particle (SRP) binds to the signal sequence and translation stops. The SRP then binds to the SRP
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