ER signal sequence
= ER signal sequence
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An <ER signal sequence> is <polypeptide chain> targeting information recognized by machinery delivering a <protein> to the <endoplasmic reticulum>. Many soluble cargoes have a removable <N-terminus> signal; membrane insertion can use internal signals. An <ER signal sequence> alone does not specify the final destination.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/The Endoplasmic Reticulum>[Molecular Biology of the Cell, 4th edition by Bruce Alberts and others; The ER Signal Sequence Is Removed from Most Soluble Proteins After Translocation; In Single-Pass Transmembrane Proteins, a Single Internal ER Signal Sequence Remains in the Lipid Bilayer as a Membrane-spanning α Helix]{ref}
For an original counting example, stipulate an unbranched precursor <polypeptide chain> of $180$ <amino acid residues> whose first $20$ <amino acid residues> form a cleaved <ER signal sequence>. With no further processing, the remaining chain has $180-20=160$ <amino acid residues> and $159$ <peptide bonds>. The initial chain had $179$ <peptide bonds>; the detached signal has $19$, so $159+19=178$ remain after one bond is cut. The check counts adjacent positions: an unbranched chain of $n$ <amino acid residues> has $n-1$ <peptide bonds>. These stipulated lengths are not a natural sequence, a measured size or evidence that a particular sequence functions as an <ER signal sequence>.