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New partners for protein kinases.

Protein post-translational modification is a critical means for the organisms to regulate their activities (Wu, 2011). Among these modifications, protein phosphorylation may be the most important one that involves in the regulation of almost all biological functions. Protein kinases are the major enzymes that are responsible for the protein phosphorylation. On the other hand, protein kinases require other proteins as the partners to assist or facilitate their kinase activities. In this JMCB issue, four papers deliver new information for us to know more about the partners for protein kinases. The review paper by Dr Delia and Buscemi’s groups systematically reviewed the research progresses of CHK2 kinase, particularly focusing on its responses to DNA damage. CHK2 is well known as a nuclear serine/threonine protein kinase involved in the spreading of DNA damage signal through a phosphorylation cascade. The authors not only discussed the classical partners for CHK2 participating in DNA damage responses, such as ATM and p53, but also summarized new CHK2 partners including proteins for cell cycle, circadian clock, and virus infection. It has been known that flow-induced T-lymphocyte migration plays an important role in mediating immune responses. In this issue, Dr Chiu’s laboratory provided new data for understanding the molecular mechanisms that caused the directionality and deformation of migratory T-lymphocytes under flow. The authors showed that protein kinase C (PKC)-d was enriched in the front of migratory cells and regulated cell protrusions through Tiam1/Rac1/calmodulin cascades. They further revealed that PKC-b, in coordination with PKC-d, regulated RhoA/Rho-associated kinase activity to control the T-lymphocyte deformation. Protein phosphorylation could play both activating and inhibitory roles in various biological processes. At the initiation stage of protein translation, phosphorylation of the eukaryotic initiation factor 2a (eIF2a) inhibits exchange of GDP and ribosome transition along the mRNA. By using a computational approach, Dr Sadler’s group generated a computational molecular model of eIF2a and the protein kinase R (PKR). The authors simulated the dynamics of the interaction between these two molecules and then predicted a novel kinase–substrate interface, which was constituted by dynamic residues in both eIF2a and PKR. They further confirmed these predications by experimentations. AMP-activated protein kinase (AMPK) has been reported to function in many physiological processes, including promoting saliva secretion in submandibular glands. In this issue, Dr Xiang and colleagues reported that claudin-4, a claudin family member as transmembrane protein of tight junction, is required for AMPK-regulated paracellular permeability in rat submandibular gland cells. The authors showed that AICAR, an AMPK agonist, induced the phosphorylation at serine residue of claudin-4 and promoted ERK1/2 phosphorylation. They concluded that claudin-4 and ERK1/2 were involved in AMPK-modulated tight junction barrier function in submandibular gland.