Keratin 20 serine 13 phosphorylation is a stress and intestinal goblet cell marker

Keratin polypeptide 20 (K20) is an intermediate filament protein with preferential expression in epithelia of the stomach, intestine, uterus, and bladder and in Merkel cells of the skin. K20 expression is used as amarker to distinguish metastatic tumor origin, but nothing is known regarding its regulation and function. We studied K20 phosphorylation as a first step toward understanding its physiologic role. K20 phosphorylation occurs preferentially on serine, with a high stoichiometry as compared with keratin polypeptides 18 and 19. Mass spectrometry analysis predicted that either K20 Ser¹³ or Ser¹⁴ was a likely phosphorylation site, and Ser¹³ was confirmed as the phospho-moiety using mutation and transfection analysis and generation of an anti-K20-phospho-Ser¹³ antibody. K20 Ser ¹³ phosphorylation increases after protein kinase C activation, and Ser¹³-to-Ala mutation interferes with keratin filament reorganization in transfected cells. In physiological contexts, K20 degradation and associated Ser¹³ hyperphosphorylation occur during apoptosis, and chemically induced mouse colitis also promotes Ser¹³ phosphorylation. Among mouse small intestinal enterocytes, K20 Ser¹³ is preferentially phosphorylated in goblet cells and undergoes dramatic hyperphosphorylation after starvation and mucin secretion. Therefore, K20 Ser¹³ is a highly dynamic protein kinase C-related phosphorylation site that is induced during apoptosis and tissue injury. K20 Ser¹³ phosphorylation also serves as a unique marker of small intestinal goblet cells.