Keratin hypersumoylation alters filament dynamics and is a marker for human liver disease and keratin mutation.
Keratin polypeptide 8 (K8) associates noncovalently with its partners K18 and/or K19 to form the intermediate filament cytoskeleton of hepatocytes and other simple-type epithelial cells. Human K8, K18, and K19 variants predispose to liver disease, whereas site-specific keratin phosphorylation confers hepatoprotection. Because stress-induced protein phosphorylation regulates sumoylation, we hypothesized that keratins are sumoylated in an injury-dependent manner and that keratin sumoylation is an important regulatory modification. We demonstrate that K8/K18/K19, epidermal keratins, and vimentin are sumoylated in vitro. Upon transfection, K8, K18, and K19 are modified by poly-SUMO-2/3 chains on Lys-285/Lys-364 (K8), Lys-207/Lys-372 (K18), and Lys-208 (K19). Sumoylation affects filament organization and stimulus-induced keratin solubility and is partially inhibited upon mutation of one of three known K8 phosphorylation sites. Extensive sumoylation occurs in cells transfected with individual K8, K18, or K19 but is limited upon heterodimerization (K8/K18 or K8/K19) in the absence of stress. In contrast, keratin sumoylation is significantly augmented in cells and tissues during apoptosis, oxidative stress, and phosphatase inhibition. Poly-SUMO-2/3 conjugates are present in chronically injured but not normal, human, and mouse livers along with polyubiquitinated and large insoluble keratin-containing complexes. Notably, common human K8 liver disease-associated variants trigger keratin hypersumoylation with consequent diminished solubility. In contrast, modest sumoylation of wild type K8 promotes solubility. Hence, conformational changes induced by keratin natural mutations and extensive tissue injury result in K8/K18/K19 hypersumoylation, which retains keratins in an insoluble compartment, thereby limiting their cytoprotective function.
Department of Molecular & Integrative Physiology, University of Michigan Medical School, Ann Arbor, Michigan 48109-5622, USA. nsnider@umich.edu
Natasha T
NT
Snider
Sujith V W
SV
Weerasinghe
Jorge A
JA
Iñiguez-Lluhí
Harald
H
Herrmann
M Bishr
MB
Omary
D000818:Animals; D015415:Biomarkers / Q000378:metabolism; D006801:Humans; D007633:Keratins / Q000235:genetics* / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008107:Liver Diseases / Q000235:genetics* / Q000378:metabolism*; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D009154:Mutation; D025841:Small Ubiquitin-Related Modifier Proteins / Q000235:genetics / Q000378:metabolism; D058207:Sumoylation; D014452:Ubiquitins / Q000235:genetics / Q000378:metabolism
D015415:Biomarkers; C117909:SUMO2 protein, human; C492189:SUMO3 protein, human; D025841:Small Ubiquitin-Related Modifier Proteins; D014452:Ubiquitins; D007633:Keratins