Death-associated protein kinase (DAPk) is a founding member of the newly classified Ser/Thr kinase family. Its members not only have significant homology in their catalytic domains, but also have functions related to cell death. It is recognized that DAPk is a tumor suppressor gene whose expression is lost in a variety of tumor types, which has stimulated interest in the kinase family and generated a large amount of literature on its function, regulation and association with disease. The DAPk family is related to several cell death-related signaling pathways and functions. In addition to cell death, other suggestions have been made.
Death-associated protein kinase (DAPk) is a Ca2+/calmodulin (CaM) -regulated Ser/Thr kinase that mediates cell death. Due to overexpression of the kinase, increased DAPk activity can lead to significant death-related cell changes, including membrane blistering, cell rounding, separation from the extracellular matrix, and formation of autophagy vesicles (1-8). In addition, DAPk activity is necessary to induce cell death through a variety of death signals, including signals from death receptors, cytokines, matrix detachment, and oncogene-induced hyperproliferation. DAPk acts as a tumor suppressor, largely because DAPk has the ability to make cells sensitive to many apoptotic signals encountered during tumorigenesis. As shown in a mouse model of tumor metastasis, DAPk inhibits cell transformation in the early stages of tumor development and also inhibits subsequent metastatic events. Interestingly, the spontaneous immortalization rate of embryonic fibroblasts derived from DAPk-/-mice was higher than that of wild type (WT). This suggests that the lack of DAPk also interferes with cellular senescence. These data have inspired many researchers to study the status of DAPk in human primary tumors. Most of these studies found that the apparent loss of DAPk expression in multiple types of tumors is primarily due to DNA methylation.
Structural characteristics of DAPk family members
DAPk, DRP-1 and ZIPk fall into one kinase subfamily because of their high degree of conservation. Common catalytic domain. However, outside this region, the size and structure of the kinases vary widely (Figure 1). Each DAPk family member includes a catalytic domain at its N-terminus, which is composed of the typical 11 subdomains found in all Ser/Thr kinases. The X-ray crystal structure of the DAPk catalytic domain has been resolved to an impressive 1.5 A and provides structural implications for DAPk's activation mechanism, its interaction with substrates, and its potential inhibitor design (18 and 10 Review). It is worth noting that the presence of two acidic amino acid clusters at the proposed substrate binding site indicates that complementary interactions with basic residues near the substrate core phosphorylation site may play a role in substrate recognition. In fact, many proposed substrates of the DAPk family have 2-3 basic residues only at the N-terminus of phosphorylated Ser or Thr.
Figure 1. The DAPk family Shown are the protein domains of the members of the DAPk family.
Mechanisms of Regulation of the DAPk Family
DAPk, DRP-1 and ZIPk are all commonly expressed in many adult mouse and rat tissues. DAPk is particularly abundant in adult and embryonic brains, especially the hippocampus. The constitutive presence of these potentially deadly proteins in normal tissues requires strict regulation, which keeps them silent in growing cells, on the one hand, and promotes effective activation of the death signal on the other. To date, most of the activation mechanisms characterized in DAPk and DRP-1 affect catalytic activity by targeting CaM self-regulation/binding. As mentioned above, to release the inhibitory effect of this domain, it is necessary to combine Ca2+ activated CaM and dephosphorylated Ser308. In fact, Derk and DRP-1 in response to certain deaths have been observed in vivo to dephosphorylate stimuli such as C6-ceramide, TNF-α, mitochondrial respiratory depression, unbound UNC5H2 and interferon (IFN) -γ. Therefore, two important cytokines are expected to activate DAPk and DRP-1: intracellular Ca2+ elevation, which is usually observed under different circumstances of programmed cell death, and is regulated by death phosphatase specifically phosphorylates Ser308. This combination should confer the specificity required to allow activation of these kinases in limited circumstances without responding to every local spike in intracellular Ca2+. Obviously, identifying putative phosphatases and how they activate during cell death will greatly help us understand how these kinases are regulated in vivo.