Regulation of cell signaling by post-translational modification of proteins.

Tony Hunter, Salk Institute for Biological Studies, La Jolla, USA

Reversible and irreversible posttranslational modifications (PTMs) are a means of increasing the complexity of the proteome; reversible PTMs are commonly used in the transmission of signals within cells in response to external stimuli. Protein phosphorylation is involved in the great majority of cellular processes, and >40,000 distinct phosphorylation events can now be detected in a single cell type. The human kinome comprises 566 protein kinases of which 480 are typical eukaryotic protein kinases (ePKs), and the remainder are atypical protein kinases (aPKs); most are Ser/Thr kinases, but there are 90 Tyr kinases. Histidine phosphorylation of proteins is becoming increasingly relevant as a regulatory mechanism, and two newly identified aPKs, NME1 and NME2, can reportedly phosphorylate histidine in target proteins. To study global histidine phosphorylation events we have generated monoclonal antibodies to noncleavable 1-pHis and 3-pHis analogues, and have begun to use these antibodies to study histidine phosphorylation in transformed cells and metastasis. Aberrant protein phosphorylation plays a role in many human diseases, and we have investigated the crosstalk between pancreatic cancer cells and pancreatic stromal cells, known as stellate cells, by proteomic analysis of secreted proteins and of tyrosine phosphorylation events induced by secreted proteins from one cell type in the other cell type. Crosstalk between PTMs is an emerging principle, and we have studied crosstalk between sumoylation and ubiquitylation in the context of TGF signaling, where the SUMO-targeted ubiquitin ligase (STUbL) Arkadia/RNF111 is required for induction of a subset of TGFβ target genes through interaction with polycomb bodies in the nucleus.