= Signal anchor
A <signal anchor> combines targeting or initiation of membrane passage with a retained membrane-spanning segment. Its orientation determines which flanking part enters the <ER lumen>; a single internal signal can yield either orientation.<Membrane Protein Insertion at the Endoplasmic Reticulum by Sichen Shao and Ramanujan S. Hegde>[Shao and Hegde, 2011; “Cotranslational Membrane Protein Topogenesis”, Figure 5]{ref}
In the figure's stipulated internal-anchor example, the <N-terminus> remains in the <cytosol> and the <C-terminus> enters the <ER lumen>. Reversing that orientation would reverse which end is protected from an externally added <protease>. Merely counting one membrane-spanning segment cannot choose between the two.
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