What could be the meaning of the red circles with letters S in them in the diagram below? I searched in the text but could not find.

From "Role of TREK-1 in Health and Disease, Focus on the Central Nervous System", Figure 2:

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1 Answer 1


It almost certainly refers to the amino acid (residue) serine. The numbers (300, 333, 351) refer to the residue numbers. The COOH at the end signifies the C-terminus of the protein (subunit).

The black P's next to it are phosphate groups; both protein kinase A and C ("PKA" and "PKC") are involved in control of protein activity precisely via phosphorylation of serine residues in proteins (amongst others).

  • 3
    $\begingroup$ There are some other hints. The letters S and E are on a line that begins with NH2 and ends in COOH. There is no element E, so S is probably no element either. There is a phosphate near the S, and serines (just like threonines and tyrosines) are targets for phosphorylation. $\endgroup$
    – Karsten
    Commented Dec 3, 2020 at 20:45

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