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Structural Requirements for Interruption of Protein Translocation Across Endoplasmic Reticulum membrane. (Kuroiwa T: Sakaquchi M; Mihara K; Omura BioChem, 1990 108: 829-34) Paraphrased abstract Rough T. J. of Co-translational translocation of proteins across the membrane of rough reticulum (RER) is interrupted by particular amino acid sequences “stop-transfer” sequences. We analyzed the structural requirements for stop tr sequences. The membrane protein interleukin 2 (IL2) is inserted in the ER membrane co-translationally; we replaced the middle portion of the IL2 molecule with different nydrophobic segments consisting of leucine, alanine, or leucine/alanine mixed clusters. Furthermore, charged amino acid residues were introduced just downstream of the hydrophobic segments. We determined the following rules for transfer sequences: (1) The number of the stopped transfers increased as the length and hydrophobicity of the inserted segment increased. (2) Proteins with hydrophobic segments followed by positive charges were more efficiently stop-translocated than those followed by negative charges. (3) If the hydrophobicity of the segment was sufficiently high, the positive charges after the segment were not essential for stop-translocation. endoplasmic which are termed ansfer Based on the three rules determined for stopping transfers in the abstract above and the properties of the amino acids (see the key below), which segment is more likely to cause a stop in translocation in each pair below? Circle the segment most likely to stop translocation in each of the 4 scenarios or put an X through both if they are equally. If you are unsure, explain on the back. or B. or C. xxxxxxXxXXXxLLLLLLLLLLLLLLLLLLLLLLLLLLKKKxxxxxxxxxxxxxxx or or Key: A = alanine (somewhat hydrophobic), L = leucine (very hydrophobic), D aspartate (negative), K lysine (positive) x any other hydrophilic amino acid

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