N-degron-mediated degradation of the proteolytically activated form of PKC-theta kinase attenuates its pro-apoptotic function

Cellular signalling cues lead to the initiation of apoptotic pathways and often result in the activation of caspases which in turn cause the generation of proteolytically generated protein fragments with new or altered functions. Mounting number of studies reveal that the activity of these proteolytically activated protein fragments can be counteracted via their selective degradation by the N-degron degradation pathways. Here, we investigate the proteolytically generated fragment of the PKC theta kinase, where we demonstrate the first report on the stability of this pro-apoptotic protein fragment. We have determined that the pro-apoptotic cleaved fragment of PKC-theta is unstable in cells because its N-terminal lysine targets it for proteasomal degradation via the N-degron degradation pathway and this degradation is inhibited by mutating the destabilizing N-termini, knockdown of the UBR1 and UBR2 E3 ligases. Tellingly, we demonstrate that the metabolic stabilization of the cleaved fragment of PKC-theta or inhibition of the N-degron degradation augments the apoptosis-inducing effect of staurosporine in Jurkat cells. Notably, we have unveiled that the cleaved fragment of PKC theta, per se, can induce apoptotic cell death in Jurkat T-cell leukemia. Our results expand the functional scope of mammalian N-degron degradation pathways, and support the notion that targeting N-degron degradation machinery may have promising therapeutic implications in cancer cells.

Automated summary

Cellular signalling leads to apoptotic pathways and caspase activation, resulting in the generation of protein fragments with new functions. These proteolytically activated protein fragments can be degraded by the N-degron degradation pathways. The stability of the pro-apoptotic fragment of PKC-theta was investigated, revealing that it is unstable due to its N-terminal lysine targeting it for proteasomal degradation. Inhibition of N-degron degradation enhanced the apoptosis-inducing effect of staurosporine.