title: The intermediate filament protein keratin 8 is a novel cytoplasmic substrate for c-Jun N-terminal kinase.
abstract: Keratins 8 (K8) and 18 are the primary intermediate filaments of simple epithelia. Phosphorylation of keratins at specific sites affects their organization, assembly dynamics, and their interaction with signaling molecules. A number of keratin in vitro and in vivo phosphorylation sites have been identified. One example is K8 Ser-73, which has been implicated as an important phosphorylation site during mitosis, cell stress, and apoptosis. We show that K8 is strongly phosphorylated on Ser-73 upon stimulation of the pro-apoptotic cytokine receptor Fas/CD95/Apo-1 in HT-29 cells. Kinase assays showed that c-Jun N-terminal kinase (JNK) was also activated with activation kinetics corresponding to that of K8 phosphorylation. Furthermore, K8 was also phosphorylated on Ser-73 by JNK in vitro, yielding similar phosphopeptide maps as the in vivo phosphorylated material. In addition, co-immunoprecipitation studies revealed that part of JNK is associated with K8 in vivo, correlating with decreased ability of JNK to phosphorylate the endogenous c-Jun. Taken together, K8 is a new cytoplasmic target for JNK in Fas receptor-mediated signaling. The functional significance of this phosphorylation could relate to regulation of JNK signaling and/or regulation of keratin dynamics.
authors.affiliation: Turku Centre for Biotechnology, University of Turku and Abo Akademi University, the Department of Biochemistry and Pharmacy, Abo Akademi University, FIN-20521, Turku, Finland.
authors.forename: Tao
authors.initials: T
authors.lastname: He
authors.forename: Andrzej
authors.initials: A
authors.lastname: Stepulak
authors.forename: Tim H
authors.initials: TH
authors.lastname: Holmström
authors.forename: M Bishr
authors.initials: MB
authors.lastname: Omary
authors.forename: John E
authors.initials: JE
authors.lastname: Eriksson
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins; D002460:Cell Line; D003593:Cytoplasm / Q000378:metabolism*; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases; D053533:Keratin-8; D007633:Keratins / Q000737:chemistry / Q000378:metabolism*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D008969:Molecular Sequence Data; D010766:Phosphorylation; D017386:Sequence Homology, Amino Acid; D012694:Serine / Q000378:metabolism; D013379:Substrate Specificity; D014157:Transcription Factors / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; C414536:FasR protein, Rhodococcus fascians; C426203:KRT8 protein, human; D053533:Keratin-8; D014157:Transcription Factors; D012694:Serine; D007633:Keratins; D048031:JNK Mitogen-Activated Protein Kinases; D020928:Mitogen-Activated Protein Kinases