Detection and identification of substrates for protein kinases: use of proteins and synthetic peptides

Assaying Bcr-Abl kinase activity and inhibition in whole cell extracts by phosphorylation of substrates immobilized on agarose beads

There is a current and increasing demand for simple, robust, nonradioactive assays of protein tyrosine kinase activity with applications for clinical diagnosis and high-throughput screening of potential molecularly targeted therapeutic agents. One significant challenge is to detect and measure the activity of specific kinases with key roles in cell signaling as an approach to distinguish normal cells from cancer cells and as a means of evaluating targeted drug efficacy and resistance in cancer cells. Here, we describe a method in which kinase substrates fused to glutathione-S-transferase and immobilized on glutathione agarose beads are phosphorylated, eluted, and then assayed to detect kinase activity. The activity of recombinant, purified c-Abl kinase or Bcr-Abl kinase in whole cell extracts can be detected with equivalent specificity, sensitivity, and reproducibility. Similarly, inhibition of recombinant c-Abl or Bcr-Abl in cells or cell extracts by imatinib mesylate and other Bcr-Abl targeted kinase inhibitors is readily assayed. This simple kinase assay is sufficiently straightforward and robust for use in clinical laboratories and is potentially adaptable to high-throughput assay formats.

Modification with a phosphorylation tag of PKA in the TraT-based display vector of Escherichia coli

We have previously developed the TraT display system to express the preS1 peptide of human hepatitis B virus (HBV) and the snake venom rhodostomin (RHO) on the surface of Escherichia coli. In this study, we modified the pT2 vector by adding a thrombin cutting site and a phosphorylation tag of protein kinase A before the multiple restriction enzyme sites. The modified vector allowed us to label the TraT fusion protein (TraT-RHO) with [32P] and to increase the detection sensitivity of TraT-RHO expression bacteria binding to and being internalized into BHK-21 cells. After the thrombin cleavage, the isotope labeled RHO could be detected in a free form. We therefore suggest that the new version of pT2 vector, pT2-KL, will facilitate to identify the counterpart of displayed peptide.

Cyclin-dependent kinases phosphorylate the adenovirus E1A protein, enhancing its ability to bind pRb and disrupt pRb-E2F complexes

The adenovirus E1A protein of 243 amino acids has been shown to affect a variety of cellular functions, most notably the immortalization of primary cells and the promotion of quiescent cells into S phase. The activity of E1A is derived, in part, from its association with various cellular proteins, many of which play important roles in regulating cell cycle progression. E1A is known to have multiple sites of phosphorylation. It has been suggested that cell cycle-dependent phosphorylation may also control some of E1A's functions. We find now that immune complexes of cyclin-dependent kinases such as cdk4, cdk2, and cdc2 are all capable of phosphorylating E1A in vitro. Additionally, the sites on E1A phosphorylated by these kinases in vitro are similar to the E1A sites phosphorylated in vivo. We have also found that a phosphorylated E1A is far more efficient than an unphosphorylated E1A in associating with pRB and in disrupting E2F/DP-pRB complexes as well. On the basis of our findings and the differences in timing and expression levels of the various cyclins regulating cdks, we suggest that E1A functions at different control points in the cell cycle and that phosphorylation controls, to some extent, its biological functions.

Two methods to avoid the effect of endogenous protein inhibitors during the assay of protein kinase C activity in tissue extracts

Using H1 as substrate the protein kinase C activity of rat liver cell sap was increased about fourfold by treatment with DEAE-cellulose at pH 7.5 at an intermediate ionic strength due to removal of protein inhibitors. The activity of cell sap from rat spleen, brain or muscle was about doubled by the same treatment. In contrast, when a specific synthetic peptide substrate was used the corresponding increase of enzyme activity was not obtained when the inhibitors were removed. This shows that this type of substrates should be preferred for reliable assays of protein kinase C in crude extracts. The possible role of the protein inhibitors for the substrate specificity of protein kinase C is briefly discussed.

Cyclic AMP-dependent protein kinase-induced vimentin filament disassembly involves modification of the N-terminal domain of intermediate filament subunits

The intermediate filament protein vimentin was phosphorylated with cAMP-dependent protein kinase under conditions that induce filament disassembly. Digestion of phosphorylated vimentin with lysine-specific endoprotease and subsequent tryptic peptide mapping indicated that a 12 kDa N-terminal fragment contained all the phosphorylation sites found in the intact molecule. Analysis of cyanogen bromide digests indicated that two phosphorylated peptides were produced, with the major 32P-labeled species representing amino acid position 14-72, and a minor 32P-labeled peptide representing amino acid positions 1-13. These results demonstrate that phosphorylation of sites within the N-terminal head domain of vimentin are associated with phosphorylation induced filament disassembly.

Poly (ADP-ribose) synthetase is phosphorylated by protein kinase C in vitro

Poly (ADP-ribose) synthetase from bovine thymus was phosphorylated effectively by protein kinase C in vitro. The phosphorylation was dependent on the activators of this kinase, Ca2+ and phospholipid. The apparent Km for the synthetase was about 8 microM, which was lower than that for histone H1. Though the synthetase was a weak substrate for Ca2+/calmodulin-dependent protein kinase II, other protein kinases, cyclic AMP-dependent and cofactor-independent protein kinases did not phosphorylate the synthetase. Phosphorylation of the synthetase by protein kinase C resulted in appreciable inhibition of the synthetase activity.

Cyclic AMP-dependent phosphorylation of high molecular mass proteins in pig thyroid cells

Four high molecular mass (H Mr) proteins were found to be phosphorylated in a cyclic AMP-dependent manner in both partially purified pig thyroid membrane fractions and in pig thyroid cells in culture. These phosphoproteins did not correspond to major cellular proteins; they were found in both soluble and particulate subfractions of homogenates from cultured thyroid cells. The molecular mass of the 4 proteins named HMr1 to HMr4 determined by polyacrylamide gel electrophoresis in the in the presence of sodium dodecylsulfate was about 310 000 for HMr1, 250 000 for HMr2, 240 000 for Hmr3 and 220 000 for HMr4. HMr1 comigrated with brain MAP1, whereas HMr3 and HMr4 had the same mobility as alpha-and beta-spectrins, respectively. The 4 high molecular mass phosphoproteins are substrates of cyclic AMP-dependent protein kinase(s) since (a) their phosphorylation was increased by cyclic AMP and not by cyclic GMP or calcium alone or calcium in the presence of calmodulin or phospholipid; (b) the effect of cyclic AMP was prevented by the thermostable inhibitor of cyclic AMP-dependent protein kinases; (c) the purified catalytic subunit of cyclic AMP-dependent protein kinases markedly phosphorylated the 4 HMr proteins. The 32P-labeling of HMr proteins using either endogenous cyclic AMP-dependent protein kinase or the purified catalytic subunit was always lower in cells cultured in the presence of TSH (reassociated in follicle-like structures) than in freshly dispersed cells or cells cultured in basal conditions (cells in monolayer). These results suggest that the 4 high molecular mass thyroid phosphoproteins represent structural components, the phosphorylation of which could vary with the cellular organization.

The role of the phosphate group for the structure of phosphopeptide products of adenosine 3',5'-cyclic monophosphate-dependent protein kinase

By c.d. studies it is shown that liver-pyruvate-kinase-related peptide substrates of cyclic AMP-dependent protein kinase have a high tendency towards non-random structures in non-aqueous media. When phosphorylated, the conformation tendencies decrease. This structural change is explained in terms of the formation of strong intrapeptide phosphate-guanidinium salt links. It is proposed that similar events occur at the catalytic site of protein kinase and that such an interaction could facilitate the removal of the phosphorylated products.