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Contains fulltext : 47822.pdf (Publisher’s version ) (Open Access), The 90-kDa ribosomal S6 kinases (RSK1-3) are important mediators of growth factor stimulation of cellular proliferation, survival, and differentiation and are activated via coordinated phosphorylation by ERK and 3-phosphoinositide-dependent protein kinase-1 (PDK1). Here we performed the functional characterization of a predicted new human RSK homologue, RSK4. We showed that RSK4 is a predominantly cytosolic protein with very low expression and several characteristics of the RSK family kinases, including the presence of two functional kinase domains and a C-terminal docking site for ERK. Surprisingly, however, in all cell types analyzed, endogenous RSK4 was maximally (constitutively) activated under serum-starved conditions where other RSKs are inactive due to their requirement for growth factor stimulation. Constitutive activation appeared to result from constitutive phosphorylation of Ser232, Ser372, and Ser389, and the low basal ERK activity in serum-starved cells appeared to be sufficient for induction of approximately 50% of the constitutive RSK4 activity. Finally experiments in mouse embryonic stem cells with targeted deletion of the PDK1 gene suggested that PDK1 was not required for phosphorylation of Ser232, a key regulatory site in the activation loop of the N-terminal kinase domain, that in other RSKs is phosphorylated by PDK1. The unusual regulation and growth factor-independent kinase activity indicate that RSK4 is functionally distinct from other RSKs and may help explain recent findings suggesting that RSK4 can participate in non-growth factor signaling as for instance p53-induced growth arrest.

Item does not contain fulltext, Transcripts of the myotonic dystrophy protein kinase (DMPK) gene, a member of the Rho kinase family, are subject to cell-type specific alternative splicing. An imbalance in the splice isoform profile of DMPK may play a role in the pathogenesis of DM1, a severe multisystemic disorder. Here, we report how structural subdomains determine biochemical properties and subcellular distribution of DMPK isoforms. A newly developed kinase assay revealed that DMPK is a Lys/Arg-directed kinase. Individual DMPK isoforms displayed comparable transphosphorylation activity and sequence preference for peptide substrates. However, DMPK autophosphorylation and phosphorylation of MYPT1 (as putative in vivo target of DMPK), were dependent on presence of an alternatively spliced VSGGG motif and the nature of the C terminus. In-gel effects of the VSGGG motif on the migration behavior of full-length kinase provide evidence for a model in which this motif mediates 3-D-conformational changes in DMPK isoforms. Finally, different C termini endow DMPK with the ability to bind to either endoplasmic reticulum or mitochondria or to adopt a cytosolic location. Our results suggest that DMPK isoforms have cell-type and location dependent substrate specificities with a role in organellar and cytoarchitectural dynamics.

The PKB (protein kinase B, also called Akt) family of protein kinases plays a key role in insulin signaling, cellular survival, and transformation. PKB is activated by phosphorylation on residues threonine 308, by the protein kinase PDK1, and Serine 473, by a putative serine 473 kinase. Several protein binding partners for PKB have been identified. Here, we describe a protein partner for PKBα termed CTMP, or carboxyl-terminal modulator protein, that binds specifically to the carboxyl-terminal regulatory domain of PKBα at the plasma membrane. Binding of CTMP reduces the activity of PKBα by inhibiting phosphorylation on serine 473 and threonine 308. Moreover, CTMP expression reverts the phenotype of v-Akt-transformed cells examined under a number of criteria including cell morphology, growth rate, and in vivo tumorigenesis. These findings identify CTMP as a negative regulatory component of the pathway controlling PKB activity.

The pleckstrin homology (PH) domain of the protooncogenic serine/threonine protein kinase PKB/Akt can bind phosphoinositides. A yeast- based two-hybrid system was employed which identified inosine-5' monophosphate dehydrogenase (IMPDH) type II as specifically interacting with PKB/Akts PH domain. IMPDH catalyzes the rate-limiting step of de novo guanosine-triphosphate (GTP) biosynthesis. Using purified fusion proteins, PKB/Akts PH domain and IMPDH associated in vitro and this association moderately activated IMPDH. Purified PKB/Akt also associated with IMPDH in vitro. We could specifically pull-down PKB/Akt or IMPDH from mammalian cell lysates using glutathione-S-transferase (GST)-IMPDH or GST-PH domain fusion proteins, respectively. Additionally, PKB/Akt and IMPDH could be co- immunoprecipitated from COS cell lysates and active PKB/Akt could phosphorylate IMPDH in vitro. These results implicate PKB/Akt in the regulation of GTP biosynthesis through its interaction with IMPDH, which is involved in providing the GTP pool used by signal transducing G-proteins.

The protooncogenic serine/threonine protein kinase PKB contains an amino-terminal pleckstrin homology (PH) domain which binds phosphatidylinositides. The PH domain, composed of ~O100 loosely conserved amino acids, is found in many proteins, including kinases, phospholipases C, GTPases, GTPase-activating proteins, GTPase-exchange factors, 'adaptor' proteins, cytoskeletal proteins, and kinase substrates. We have developed an expression system in Escherichia coli that can produce large quantities of a soluble form of the PKB PH domain and have purified it to apparent homogeneity. Expression of the PKB PH domain as a (His)6-tagged fusion with the addition of 3 lysines at the carboxyl-terminus facilitated the production of soluble protein. Induction of expression at 24°C as opposed to 37°C also significantly increased solubility of the PH domain. Large-scale purification was easily achieved by exploiting the (His)6 tag and the high isoelectric point of the protein attributable to the additional 3 carboxyl-terminal lysines.

The influence of inositol phosphates and phosphoinositides on the α isoform of the RAC-protein kinase B (RAC/PKB) was studied using purified wild type and mutant kinase preparations and a recombinant pleckstrin homology (PH) domain. Binding of inositol phosphates and phosphoinositides to the PH domain was measured as the quenching of intrinsic tryptophan fluorescence. Inositol phosphates and D3-phosphorylated phosphoinositides bound with affinities of 1-10 μM and 0.5 μM, respectively. Similar values were obtained using RAC/PKB expressed and purified from baculovirus-infected Sf9 cells in the fluorescence assay. The influence of synthetic dioctanoyl derivatives of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate on the activity of RAC/PKB purified from transfected COS- 1 cells was studied. Phosphatidylinositol 3,4,5-trisphosphate was found to inhibit the RAC/PKB kinase activity with half-maximal inhibition at 2.5 μM. In contrast, phosphatidylinositol 3,4-bisphosphate stimulated kinase activity (half-maximal stimulation at 2.5 μM). A mutant RAC/PKB protein lacking the PH domain was not affected by D3-phosphorylated phosphoinositides. These results demonstrate that the PH domain of RAC/PKB binds inositol phosphates and phosphoinositides with high affinity, and suggest that the products of the phosphatidylinositide 3-kinase can act as both a membrane anchor and modulator of RAC/PKB activity. The data also provide further evidence for a link between phosphatidylinositide 3-kinase and RAC/PKB regulation.

A diverse array of molecules involved in signal transduction have recently been recognised as containing a new homology domain, the pleckstrin homology (PH) domain. These include kinases (both serine/threonine and tyrosine specific), all currently known mammalian phospholipase Cs, GTPases, GTPage‐activatng proteins, GTpace‐exchange factors, “adapter” proteins, cyotskeletal proteins, and kinase substrates. This has sparked a new surge of research into elucidating its sturcture and function. The NMR solution structure of the PH domains of β‐spectrin and pleckstrin (the N‐terminal domain) both display a core consisting of seven anti‐parallel β‐sheet strands. The carboxy terminus is folded into a long α‐helix. The molecule is electrostatically polarised and contains a pocket which may be involved in the inding of a ligand. The PH domain overall topological relatedness to the retinoid inding protein family of molecules would suggest a lipid ligand could bind to this pocket. the prime function of the PH domain still remains to be elucidated. However, it has been shown to be important in signal transduction, most probably by mediating protein‐protein interactions. An extended PH domain of the β‐adrenergic receptor kinase (βARK), as well as that of several other molecules, can bind to βγ subunits of the heterotrimeric G‐proteins. The possibility that the PH domain, which is found in so many signalling molecules, being generally inovolved in βγ binding site appear to be concomitant in βARK, detailed analysis indicates that the PH domain is not generally a βγ binding domain. Thus, the race is on to find the ligands of each PH domain and determine a common nature to their interaction.