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Vol. 13, Issue 6, 1857-1870, June 2002

Type II Keratins Are Phosphorylated on a Unique Motif during Stress and Mitosis in Tissues and Cultured Cells

Diana M. Toivola,^* Qin Zhou,^* Luc S. English, and M. Bishr Omary^*

 ^*Department of Medicine, Veterans Affairs Palo Alto Health Care System, Palo Alto, California 94304 and Digestive Disease Center, Stanford University School of Medicine, Stanford, California 94305; and  Department of Chemical Biology, Universite du Quebec a Trois-Rivieres, Quebec, Trois-Rivieres, G9A-5H7, Canada

Epithelial cell keratins make up the type I (K9-K20) and type II (K1-K8) intermediate filament proteins. In glandular epithelia, K8 becomes phosphorylated on S73 (⁷¹LLpSPL) in human cultured cells and tissues during stress, apoptosis, and mitosis. Of all known proteins, the context of the K8 S73 motif (LLS/TPL) is unique to type II keratins and is conserved in epidermal K5/K6, esophageal K4, and type II hair keratins, except that serine is replaced by threonine. Because knowledge regarding epidermal and esophageal keratin regulation is limited, we tested whether K4-K6 are phosphorylated on the LLTPL motif. K5 and K6 become phosphorylated in vitro on threonine by the stress-activated kinase p38. Site-specific anti-phosphokeratin antibodies to LLpTPL were generated, which demonstrated negligible basal K4-K6 phosphorylation. In contrast, treatment of primary keratinocytes and other cultured cells, and ex vivo skin and esophagus cultures, with serine/threonine phosphatase inhibitors causes a dramatic increase in K4-K6 LLpTPL phosphorylation. This phosphorylation is accompanied by keratin solubilization, filament reorganization, and collapse. K5/K6 LLTPL phosphorylation occurs in vivo during mitosis and apoptosis induced by UV light or anisomycin, and in human psoriatic skin and squamous cell carcinoma. In conclusion, type II keratins of proliferating epithelia undergo phosphorylation at a unique and conserved motif as part of physiological mitotic and stress-related signals.

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Corresponding author.

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Molecular Biology of the Cell
Vol. 13, 1857-1870, June 2002
Copyright © 2002 by The American Society for Cell Biology

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