RSK and MSK in MAP kinase signalling

Abstract

RSK [for 90 kDa ribosomal S6 kinase (p90rsk); also known as MAPKAP-K1] and MSK (for mitogen- and stress-activated protein kinase) constitute a family of protein kinases that mediate signal transduction downstream of MAP kinase cascades. RSK is activated by MAP kinases of the extracellular signal-regulated kinase (ERK) family in response to growth factors, many polypeptide hormones, neurotransmitters, chemokines and other stimuli. MSK is also activated by ERK in response to such stimuli, but in addition, it can be activated by p38 family MAP kinases in response to various cellular stress stimuli and pro-inflammatory cytokines/factors. In mammals, four RSK genes (RSK1-RSK4) and two MSK genes (MSK1 and MSK2) have been identified. RSK and MSK are ubiquitously expressed and many cell types express several members of each family. At present, little is known about specific and overlapping functions of the individual RSK/MSK family members. Orthologues of RSK and MSK have been described in D. melanogaster and C. elegans, but not in yeast or plants.RSK and MSK each contain two kinase domains connected by a regulatory linker sequence. The N-terminal kinase domain (NTK) belongs to the AGC kinase family and is responsible for phosphorylation of substrates. The C-terminal kinase domain (CTK) belongs to the CamK family and its only known function is activation of NTK. The activation mechanism of RSK involves phosphorylation at four major sites (Dalby et al., 1998). First, ERK, bound to a C-terminal MAP-kinase-docking site, phosphorylates the linker at Ser369 and the activation loop of CTK at Thr577 (human RSK2 numbering). Phosphorylation of Thr577 activates CTK, which thereafter phosphorylates Ser386, which is located within a hydrophobic motif of aromatic residues in the linker. Phosphorylation of Ser386 generates a docking site that recruits 3-phosphoinositide-dependent protein kinase 1 (PDK1) and stimulates its activity five times, allowing PDK1 to phosphorylate NTK at Ser227 in the activation loop (Frodin et al., 2000). Thus, despite its name, PDK1 does not depend on phosphoinositide 3-kinase (PI3-K) activity for activation of RSK. After dissociation of PDK1, phosphoSer386 binds a phosphate-binding site in NTK, which enables the aromatic residues of the hydrophobic motif to bind and stabilize a nearby hydrophobic pocket, partly formed by the so-called αC-helix. The ordered αC-helix then cooperates with phosphoSer227 in the activation loop to stabilize NTK in an active conformation, resulting in synergistic stimulation of kinase activity (Frodin et al., 2002). The phosphorylation of Ser369 stimulates the catalytic activity of NTK two- to threefold, but the mechanism is unknown. Inactivation of RSK involves NTK-catalyzed phosphorylation Ser737. This event decreases the affinity of ERK for RSK, which helps prevent reactivation of RSK after dephosphorylation of the activating sites. RSK4 is somewhat unusual in that it has very low expression levels and appears to be fully phosphorylated and activated in unstimulated cells for reasons not entirely clear (Dümmler et al., 2005). The activation mechanism of MSK follows a similar scheme, except for some important differences (Deak et al., 1998; McCoy et al., 2005). First, the MAP-kinase-docking site of MSK can interact with both ERK and p38. Second, in addition to the serine in the hydrophobic motif, CTK also phosphorylates the activation loop of NTK as well a site in the linker, not conserved in RSK, which contributes to full activation. RSK is abundant in the cytoplasm and, upon activation, a fraction of the cytosolic RSK molecules appears to translocate to the nucleus by an unknown mechanism. MSK is restricted to the nucleus.Identification of bona fide substrates and functions of RSK and MSK has been difficult owing to the lack of specific inhibitors. The poster shows a number of likely substrates, of which about 70% have been thoroughly validated by evidence from knockout/knockdown cells, semi-specific inhibitors and correlation between kinase activation and substrate phosphorylation, as exemplified by LKB1 (Sapkota et al., 2001). The poster also indicates cellular processes thought to be regulated by RSK or MSK via these substrates, but only in about 30% of the cases has the phosphorylation event been shown to be rate limiting. RSK and MSK phosphorylate serine residues (threonine phosphorylation has been reported only twice) in variations of the motifs R/KxRxxS and RRxS (or RKS), respectively, but these are not fully defined. The RSK-phosphorylation motif overlaps one of the phosphoSer motif recognized by 14-3-3 proteins, and in NHE1, BAD and p27Kip1, 14-3-3 recruitment appears to be important for modulation of function resulting from phosphorylation by RSK. Experiments using overexpressed, constitutively active mutants of RSK and MSK have helped identify possible cellular functions regulated by these kinases, but must be supported by other evidence, because most AGC ki