In the classical three-tiered MAPK pathways, MAP kinase kinase kinase (MAP3K) activates a MAP2K, which in turn induces dual threonine and tyrosine phosphorylation and subsequent activation of a MAPK. The phosphorylation of MAPKs can be dephosphorylated and inactivated by dual-specificity phosphatases. While the upstream activators of MAP3Ks are often small GTP-binding proteins, but in some pathways, MAP3K can be activated by another kinase, which therefore is named as a MAP kinase kinase kinase kinase (MAP4K). The structures of MAP4Ks share a high degree of similarity. They are all composed of an N-terminal kinase domain, proline-rich motifs, and a C-terminal citron-homology domain. To date, six MAP4Ks includes HPK1 (MAP4K1), GCK (MAP4K2), GLK (MAP4K3), HGK/NIK (MAP4K4), KHS/GCKR(MAP4K5), and MINK(MAP4K6). All of these MAP4Ks belong to the mammalian Ste20-like serine/threonine kinase family, which are homologs of the yeast sterile20 protein (Ste20p), a putative MAP4K that activates a MAP3K in the yeast pheromone signaling pathway. 
These mammalian Ste20-like kinases are divided into two subfamilies based on the domain structures: the p21-activated kinases (PAKs) and the germinal center kinases (GCKs). Among the GCK subfamily, several of them can activate the MAP3K kinase cascade, leading to JNK activation. MAP4Ks control many signaling pathways, which could regulate different aspects of immune cell functions. MAP4Ks (GCK, KHS, and MINK) are also involved in T-cell activation and immune regulation. Further characterization of MAP4Ks’ functions in different types of immune cells should provide significant insight into the regulation of the immune system. HPK1 downregulation or GLK overexpression in T cells may cause autoimmune diseases. HGK downregulation in T cells could cause insulin resistance and type 2 diabetes. Moreover, HPK1, GLK, and HGK are also involved in tumorigenesis. These findings suggest that MAP4Ks are novel biomarkers and/or drug targets for treating these diseases.


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