Protein kinase C alpha actively downregulates through caveolae-dependent traffic to an endosomal compartment

Receptor desensitization occurs through receptor internalization and target ing to endosomes, a prerequisite for sorting and degradation. Such traffick ing processes may not be restricted to membrane associated receptors but ma y also play an important role in the downregulation of cytoplasmic transduc ers such as protein kinase C (PKC), It is demonstrated here that acute TPA exposure induces the transport of activated PKC alpha from the plasma membr ane to endosomes, This process requires PKC activity and catalytically comp etent PKC can even promote a similar process for a truncated regulatory dom ain PKC alpha protein. It is established that PKC alpha is targeted to the endosome compartment as an active kinase, where it colocalizes with annexin I, a substrate of PKC, Thus, PKC alpha downregulation shares features with plasma membrane associated receptor sorting and degradation. However, it i s shown that PKC alpha delivery to the endosome compartment is not a Rab5 m ediated process in contrast to the well characterised internalisation of th e transferrin receptor, An alternative route for PKC alpha is evidenced by the finding that the cholesterol binding drugs nystatin and filipin, known to inhibit caveolae mediated trafficking, are able to block PKC alpha traff ic and down regulation. Consistent with this, the endosomes where PKC alpha is found also contain caveolin, It is concluded that the initial step in d esensitisation of PKC alpha involves active delivery endosomes via a caveol ae mediated process.