Inhibition of the c-Jun N-terminal kinase/AP-1 and NF-κB pathways by PICOT, a novel protein kinase C-interacting protein with a thioredoxin homology domain

Protein kinase C-theta (PKC theta) is a Ca2+-independent PKC isoform that is selectively expressed in T lymphocytes (and muscle), and is thought to play an important role in T cell receptor-induced activation. To gain a better understanding of the function and regulation of PRC theta, we have employed the yeast two-hybrid system to identify PKC theta-interacting proteins. We report the isolation and characterization of a cDNA encoding a novel 335-amino acid (37.5-kDa) PKC theta-interacting protein termed PICOT (for (P) under bar KC-(i) under bar nteracting (c) under bar ousin (o) under bar f (t) under bar hioredoxin). PICOT is expressed in various tissues, including in T cells, where it colocalizes with PKC theta. PICOT displays an N-terminal thioredoxin homology domain, which is required for the interaction with PKC, Comparison of the unique C-terminal region of PICOT with expressed sequence tag data bases revealed two tandem repeats of a novel domain that is highly conserved from plants to mammals. Transient overexpression of full-length PICOT (but not its N- or C-terminal fragments) in T cells inhibited the activation of c-Jun N-terminal kinase (but not extracellular signal-regulated kinase), and the transcription factors AP-1 or NF-kappa B. These findings suggest that PICOT and its evolutionary conserved homologues may interact with PKC-related kinases in multiple organisms and, second, that it plays a role in regulating the function of the thioredoxin system.