Ser221
Ser221 is a residue designation used in protein sequence annotations to refer to the serine amino acid occupying the 221st position in a polypeptide chain. Because residue numbering is protein-specific, Ser221 occurs in many different proteins; the same numeric label identifies distinct serine residues in different molecules. The functional significance of Ser221 depends on its surrounding sequence and structural context; in many cases, the serine can be a site for post-translational modification, most commonly phosphorylation by serine/threonine kinases, which can modulate catalytic activity, protein interactions, subcellular localization, or stability. If Ser221 is phosphorylated, it is often written as pS221 or pSer221 in experimental reports. Detection typically relies on mass spectrometry-based phosphoproteomics or, when available, phospho-specific antibodies; researchers may also investigate Ser221 by mutating it to alanine (S221A) or aspartate/glutamate (S221D/E) to mimic or block phosphorylation and assess functional effects.
The exact position can shift in annotated proteins that undergo signal peptide cleavage or processing, so numbering