Regulation of kinase activity

SH3 domains specifically regulate kinase activity of expressed Src family proteins

The Src homology 2 (SH2) and Src homology 3 (SH3) domain are approximately 50% conserved in various Src family kinase members. Several lines of evidence suggest that in Src these domains are sequence motifs that direct substrate recognition, regulate kinase activity, or control subcellular localization. We sought to investigate the function of the homology domains in human Lyn, and to determine whether the differences between various SH3 domains affect function. To do this, we generated variant forms of Lyn lacking SH2 and SH3 domains, and created chimeras in which the SH3 domains in human c-Src and Lyn were replaced with SH3 domains from other family members. In contrast to similar deletions in Src, forms of Lyn lacking SH2 or SH3 had decreased kinase activity. The SH3 chimeras all had individual characteristics. Insertion of the Blk SH3 domain into Lyn restored kinase activity, while insertion of the Fyn or Src SH3 into Lyn enhanced the kinase activity 2-3-fold. Insertion of the Lyn SH3 into Src also doubled kinase activity. Expression of the Lyn-Src SH3 chimera in mammalian cells induced cell transformation. This study 1) demonstrates that the regulation of Lyn is different than Src, and 2) provides new evidence that despite their homology, there are important functional differences between the SH3 domains of the various Src family members.

Substrate phosphorylation capacities of the major tyrosine protein kinase from the human promyelocytic cell line, HL-60

The major tyrosine protein kinase, HPK40, isolated from HL-60, the preparation of which is described elsewhere (Ernould, A.P., Ferry, G., Barret, J.M., Genton, A. and Boutin, J.A., Eur. J. Biochem., 214, 503-514), was investigated as to its specificity on a number of peptides and proteins. It was found that HPK40 can phosphorylate histones (except histone H4), casein, acid-treated enolase, actin and tubulin but not calmodulin. Phosphorylation specificity of HPK40 was investigated using over a hundred peptidic structures. HPK40 is not related to the 'src' family and does not phosphorylate efficiently either the tetrapeptide NEYT derived from the pp60src autophosphorylation domain or the corresponding peptide RRsrc, RRLIED-NEYTARG. VALYDYESR from the SH3 domain of pp60c-src is recognized as a substrate with a high phosphorylation level. DEDYIQD, derived from the phosvitin/casein kinase II, was also highly phosphorylated. In order to determine the minimal recognition sequence of HPK40, the phosphorylation of about 60 dito tetrapeptides was investigated. Some of the tetrapeptides, such as *EEYE and NEYE, were well phosphorylated. Even some tripeptides, such as EYE, DYM, TYS and KYE, were recognized by HPK40, while none of the tested dipeptides was recognized as substrate. Sequences of peptides from DRVYHPF (angiotensin), LEEEEEAYGWMDF (minigastrin) and QEEYSAM (from H-ras1) were examined as substrates. The presence of one or several acidic residues on the N alpha-side of tyrosine residue was identified as the only apparently favorable determinant.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemistry of the Src protein-tyrosine kinase: regulation by SH2 and SH3 domains

pp60c-Srs (c-Src) is the prototype for a family of cytoplasmic protein-tyrosine kinases involved in the control of signal transduction. In addition to the enzymatic kinase domain, c-Src has several noncatalytic domains which regulate Src tyrosine kinase activity in both a positive and a negative fashion. Phosphorylation of c-Src at Tyr527 in the noncatalytic C-terminal tail is a key mechanism for repression of c-Src tyrosine kinase activity. This inhibitory phosphorylation is apparently catalyzed by another cytoplasmic tyrosine kinase (Csk). Recent evidence suggests that the c-Src SH2 domain participates in this phosphorylation-dependent repression of kinase activity through an intramolecular association with the phosphotyrosine-containing C-terminus. The SH3 domain of c-Src also negatively regulates c-Src tyrosin kinase activity, although the mechanism is as yet unknown. However, in the background of constitutively active transforming Src variants, such as a c-Src mutant with an amino acid substitution eliminating Tyr527 (527F c-Src) or the retroviral oncogene v-src product pp60v-src (v-Src), both the SH2 and SH3 domains contribute positively to the enzymatic and biological activities of the Src tyrosine kinase through interactions with Src substrates and/or cellular regulators.

Purification and characterization of the major tyrosine protein kinase from the human promyelocytic cell line, HL60

The major tyrosine protein kinase from HL60 (a human non-differentiated promyelocytic cell line) has been purified almost to homogeneity as judged by silver-stained SDS/PAGE. The procedure involved four chromatographic steps: DEAE-Sepharose, casein-agarose, cibacron-blue--agarose and hexyl-agarose. The purification resulted in more than 1000-fold enrichment in angiotensin II phosphorylation activity. A gel-sizing experiment, labeling with [35S]ATP[gamma s] and autophosphorylation of the enzyme in the presence of [gamma-32P]ATP, all led to the identification of a single protein species with a molecular mass of about 40 kDa. Western blot experiments showed that this protein does not belong to the src family and is not related to the abl and fes oncogene products. Phosphorylation of angiotensin II and casein by this 40-kDa human promyelocytic kinase was stimulated by high ionic strength especially from class IA metal salts. The Km for ATP was 2 microM and the Vmax 3.1 nmol.min-1.mg-1 using angiotensin II as a substrate. The kinase requires the presence of either Mn2+ or Mg2+ for full activity and utilizes ATP or dATP but not GTP as phosphate donor. Based on numerous biochemical observations, it was possible to demonstrate that kinase is different from any other tyrosine protein kinases described in the literature. This 40-kDa protein was used as a molecular tool for testing some tyrosine protein kinase inhibitors described in the literature. It is one of the rare tyrosine protein kinases purified from human cancer cells to date.

Small deletion in v-src SH3 domain of a transformation defective mutant of Rous sarcoma virus restores wild type transforming properties

RSV mutant virus PA101T was obtained while assaying the tumorigenicity of parental PA101 virus in chickens. PA101 is a transformation defective mutant of RSV which has a low src kinase activity. However, PA101 retained a temperature-sensitive ability to induce sustained proliferation of neuroretina cells. PA101T appeared as a wild-type phenotype revertant of PA101. Molecular cloning and sequencing of PA101T showed that this reversion is due to additional mutations in PA101 src gene. These mutations are a deletion eliminating three amino acids in the N-terminal region of SH3 domain and mutation of Ala 426 to Val. Analysis of the properties of chimeric src genes associating either half of PA101T with the complementary regions of PA101 or wild-type virus showed that the N-terminal moiety of PA101T src, which contains the deletion, confers wild-type transforming properties, whereas its C-terminal moiety, which contains single amino acid mutation, confers a partially temperature-sensitive phenotype. These results are consistent with other reports showing that mutations or deletions in this region of SH3 activate the transforming potential of c-src. They support the hypothesis that the N-terminal region of SH3 interacts with a cellular negative regulator of src activity.

SH3--an abundant protein domain in search of a function

Src-homology 3 is a small protein domain of about 60 amino acid residues. It is probably made of beta-sheets. SH3 is present in a large number of eukaryotic proteins which are involved in signal transduction, cell polarization and membrane-cytoskeleton interactions. Here we review its occurrence and discuss possible functions of this domain.

Effects of SH2 and SH3 deletions on the functional activities of wild-type and transforming variants of c-Src

The amino-termina, noncatalytic half of Src contains two domains, designated the Src homology 2 (SH2) and Src homology 3 (SH3) domains, that are highly conserved among members of the Src family of tyrosine kinases. The SH2 domain (which can be further divided into the B and C homology boxes) and the SH3 domain (also referred to as the A box) are also found in several proteins otherwise unrelated to protein tyrosine kinases. It is believed that these domains are important for directing specific protein-protein interactions necessary for the proper functioning of Src. To determine the importance of the SH2 and SH3 domains in regulating the functions of c-Src, we evaluated mutants of c-Src lacking the A box (residues 88 to 137), the B box (residues 148 to 187) or the C box (residues 220 to 231). Each of these deletions caused a 14- to 30-fold increase in the in vitro level of kinase activity of c-Src. Chicken embryo fibroblasts expressing the deletion mutants displayed a transformed cell morphology, formed colonies in soft agar, and contained elevated levels of cellular phosphotyrosine-containing proteins. Src substrates p36, p85, p120, p125, the GTPase-activating protein (GAP), and several GAP-associated proteins were phosphorylated on tyrosine in cells expressing the A, B, or C box deletion mutant. p110 was highly phosphorylated in cells expressing the C box mutant, was weakly phosphorylated in cells expressing the B box mutant, and was not phosphorylated in cells expressing the A box mutant. Expression of the mutant proteins caused a reorganization of the actin cytoskeleton similar to that seen in v-Src-transformed cells. In addition, deletion of the A, B, or C box did not diminish the transforming or enzymatic activity of an activated variant of c-Src, E378G. These data indicate that deletion of the A, B, or C homology box causes an activation of the catalytic and transforming potential of c-Src and that while these mutations caused subtle differences in substrate phosphorylation, the homology boxes are not required for many of the phenotypic changes associated with transformation by Src.

Effects of SH2 and SH3 deletions on the functional activities of wild-type and transforming variants of c-Src

The amino-termina, noncatalytic half of Src contains two domains, designated the Src homology 2 (SH2) and Src homology 3 (SH3) domains, that are highly conserved among members of the Src family of tyrosine kinases. The SH2 domain (which can be further divided into the B and C homology boxes) and the SH3 domain (also referred to as the A box) are also found in several proteins otherwise unrelated to protein tyrosine kinases. It is believed that these domains are important for directing specific protein-protein interactions necessary for the proper functioning of Src. To determine the importance of the SH2 and SH3 domains in regulating the functions of c-Src, we evaluated mutants of c-Src lacking the A box (residues 88 to 137), the B box (residues 148 to 187) or the C box (residues 220 to 231). Each of these deletions caused a 14- to 30-fold increase in the in vitro level of kinase activity of c-Src. Chicken embryo fibroblasts expressing the deletion mutants displayed a transformed cell morphology, formed colonies in soft agar, and contained elevated levels of cellular phosphotyrosine-containing proteins. Src substrates p36, p85, p120, p125, the GTPase-activating protein (GAP), and several GAP-associated proteins were phosphorylated on tyrosine in cells expressing the A, B, or C box deletion mutant. p110 was highly phosphorylated in cells expressing the C box mutant, was weakly phosphorylated in cells expressing the B box mutant, and was not phosphorylated in cells expressing the A box mutant. Expression of the mutant proteins caused a reorganization of the actin cytoskeleton similar to that seen in v-Src-transformed cells. In addition, deletion of the A, B, or C box did not diminish the transforming or enzymatic activity of an activated variant of c-Src, E378G. These data indicate that deletion of the A, B, or C homology box causes an activation of the catalytic and transforming potential of c-Src and that while these mutations caused subtle differences in substrate phosphorylation, the homology boxes are not required for many of the phenotypic changes associated with transformation by Src.

Importance of oncogene research to the cancer clinician

For many years a genetic basis has been postulated for cancers and leukaemias. However, this concept has now been strengthened by the discovery of a group of genes (oncogenes) which are specifically associated with neoplasia. These genes have been identified by studies of animal oncogenic viruses, by in vitro transformation studies of mouse fibroblasts, and by molecular analyses of human tumour cells. The oncogenes are altered forms of normal genes (proto-oncogenes) whose protein products are thought to be involved in the regulation of cell proliferation. It appears that the alteration of proto-oncogenes to form oncogenes results in the inappropriate production of normal proteins or in the production of abnormal proteins. It is suggested, therefore, that the presence of such proteins would allow cells to escape from normal cell growth regulation, and as a result produce uncontrolled proliferation. Oncogene research has to date been primarily concerned with identifying oncogenes and assessing their importance in the development of a number of malignancies. Such research is therefore allowing us to build up a framework of genetic changes which define the development of each type of tumour or leukaemia. Furthermore, information concerning the oncogenes is now beginning to be applied diagnostically to help in determining predisposition of an individual to disease, in aiding the accurate staging of disease, in indicating prognosis and in developing markers to detect residual disease after therapy. It is also possible that in the future we may be able to develop new anticancer therapies, for example, based on oncogene protein inhibitors, or anti-oncogene protein antibodies or even gene therapy.

Cyclic nucleotide-dependent protein kinases

The actions of several hormones and neurotransmitters evoke signal transduction pathways which rapidly elevate the cytosolic concentrations of the intracellular messengers, cAMP and cGMP. The cyclic-nucleotide dependent protein kinases, cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG), are the major intracellular receptors of cAMP and cGMP. These enzymes become active upon binding respective cyclic nucleotides and modulate a diverse array of biochemical events through the phosphorylation of specific substrate proteins. The focus of this review is to describe the progress made in understanding the structure and function of both PKA and PKG.