Okadaic acid-sensitive protein phosphatases dephosphorylate MARCKS, a major protein kinase C substrate

The myristoylated alanine-rich C kinase substrate (MARCKS) undergoes a rapid and, in certain circumstances, transient increase in phosphorylation in response to stimuli that activate protein kinase C. We have investigated the protein-serine/threonine phosphatase activity responsible for reversing the phosphorylation of MARCKS. In cell-free assays, protein phosphatases 1, 2A and 2C (PP1, PP2A and PP2C) all dephosphorylate recombinant MARCKS or a synthetic peptide based on its phosphorylation site domain. In intact Swiss 3T3 cells, okadaic acid, a specific inhibitor of PP1 and PP2A, had little effect on MARCKS phosphorylation on its own, but largely prevented the dephosphorylation of MARCKS that occurred following activation of protein kinase C by bombesin with subsequent receptor blockade. These results indicate that although the dephosphorylation of MARCKS can be mediated by PP2C in vitro, this protein is dephosphorylated by okadaic acid-sensitive phosphatases in the intact cell.

Nucleotide sequence, expression, and chromosomal mapping of Mrp and mapping of five related sequences

We isolated and characterized a genomic clone for the mouse MARCKS-related protein, or MRP, also known as F52 and MacMARCKS. A 3699-bp plasmid contained 407 bp of 5'-flanking region, 186 bp of 5'-untranslated region, 600 bp of protein coding region, 784 bp of a single intron, 746 bp of 3'-untranslated region, and 976 bp of 3'-flanking region. The position of the single intron was identical to the intron position in all known MARCKS mRNAs. When the plasmid containing the genomic sequences was transfected into fibroblasts lacking endogenous Mrp expression, the 407 bp of promoter conferred high-level expression of the full-length, spliced mRNA. The putative promoter was therefore functional; however, despite tissue-specific regulation and transcriptional induction in some cells in a manner similar to that seen with MARCKS expression, the promoters were highly dissimilar at the level of primary sequence (37% identity over 407 bp). Mrp mapped to a position on mouse chromosome 4 that was closely linked to the Lck locus. Numerous additional species that hybridized to the MRP cDNA were noted on Southern blotting of mouse genomic DNA. Five related loci were labeled Mrp-rs1 through Mrp-rs5 (for Mrp-related sequences) and were mapped to mouse chromosomes 10, 17, 15, 13, and X, respectively. Three of these related sequences have been cloned, and all appear to represent pseudogenes.

Severely decreased MARCKS expression correlates with ras reversion but not with mitogenic responsiveness

Phorbol ester-inducible phosphorylation of MARCKS, the '80-kDa' substrate of protein kinase C, was undetectable in several phenotypically dominant, non-transformed revertants independently derived from the ras-transformed cell line NIH3T3 DT-ras. Extremely low expression of MARCKS protein accounted for this apparent lack of phosphorylation. MARCKS-encoding mRNA levels were correspondingly decreased relative to normal and ras-transformed cells in all four ras revertant cell lines studied: C-11 and F-2, derived by 5-azacytidine treatment and selection with ouabain; CHP 9CJ, derived by ethylmethane sulfonate mutagenesis and selection with cis-hydroxy-L-proline; and 12-V3, derived by transfection with the human Krev-1 gene. However, re-expression of MARCKS after transfection of a cloned MARCKS cDNA into the C-11 ras revertant cells was not sufficient to induce retransformation. In fact, no significant difference in sensitivity to mitogenic stimulation by phorbol esters was observed among several cell lines expressing widely varying levels of MARCKS. This evidence argues against a direct role for MARCKS in mitogenic signaling. However, the strong correlation between attenuation of MARCIS expression and phenotypically dominant ras reversion suggests that a common negative regulatory mechanism might be responsible for both effects, presenting a potentially useful strategy for identifying factors involved in transducing the ras signal.

Regulation of peptide-calmodulin complexes by protein kinase C in vivo

We used the freshwater protozoan Paramecium tetraurelia to investigate the potential regulation by protein kinase C of calmodulin interactions with binding peptides in intact cells. In these organisms, an action potential results in membrane depolarization and a period of backward swimming; repolarization and a return to forward swimming requires the presence of normal calmodulin. We postulated that injection of high-affinity calmodulin binding peptides might interfere with repolarization and thus prolong the period of membrane depolarization. Synthetic peptides spanning the protein kinase C phosphorylation site/calmodulin-binding domains of the myristoylated alanine-rich C-kinase substrate (MARCKS) and the MARCKS-related protein (also known as F52 or MacMARCKS) were injected into cells; these caused a 2- to 3-fold increase in the duration of backward swimming. Similar changes were seen with two other calmodulin-binding peptides. This behavioral response could be prevented by coinjecting calmodulin. Activation of Paramecium protein kinase C with an active phorbol ester completely reversed (within 3 min) the behavioral effects of the normal MARCKS and MARCKS-related protein peptides. Injection of a nonphosphorylatable peptide, in which alanines were substituted for serines, resulted in the usual behavioral response; however, this was not reversed by phorbol ester treatment. The corresponding aspartate-substituted peptide, which has a 10-fold lower affinity for calmodulin, did not prolong backward swimming. These data suggest that these peptides can form complexes with calmodulin at the calcium concentrations that prevail in intact Paramecium cells and that such complexes can be disrupted by protein kinase C phosphorylation of the peptides.

Membrane association of the myristoylated alanine-rich C kinase substrate (MARCKS) protein appears to involve myristate-dependent binding in the absence of a myristoyl protein receptor

The myristoylated alanine-rich C kinase substrate, or MARCKS protein, has been implicated in several cellular processes, yet its physiological function remains unknown. We have studied the molecular basis of its membrane association in a cell-free system in order to help elucidate its regulation and function. First, we showed that the MARCKS protein incorporated [3H]myristate when its mRNA was translated in vitro in reticulocyte lysates. The myristoylated protein bound rapidly to freshly fractionated cell membranes, while a nonmyristoylated mutant associated to a much lesser extent (< 15% of wild type). To determine whether this binding was due to a specific cytoplasmic-face protein "receptor," as is seen with pp60v-src, we pretreated the membranes in several ways. Prior treatment of membranes with heat (100 degrees C for 3 min) or trypsin did not affect subsequent MARCKS binding. Binding was markedly decreased in 50 mM EDTA, 0.5 M NaCl, or 1.0% Triton X-100; it was restored to normal after removal of the NaCl and EDTA but was still decreased after removal of the Triton X-100. These findings argued against the existence of a protein receptor for the MARCKS protein on cellular membranes. Finally, MARCKS protein phosphorylated in vitro with protein kinase C bound to the cell membranes to the same extent as the nonphosphorylated protein; this binding was also unaffected by an excess of a synthetic peptide corresponding to the phosphorylation site domain of the protein. We conclude that, at least in this in vitro system, the membrane association of the MARCKS protein is primarily dependent on the amino-terminal myristate moiety and does not appear to involve a specific cytoplasmic-face protein receptor.

Chromosomal mapping of the human (MACS) and mouse (Macs) genes encoding the MARCKS protein

The myristoylated, alanine-rich C-kinase substrate, or MARCKS protein, is a major cellular substrate for protein kinase C that is also a high-affinity calmodulin-binding protein. In addition, it is the prototype of a small family of myristoylated, calmodulin-binding protein kinase C substrate proteins. We isolated a phage clone from a mouse genomic library that spanned the entire coding sequence of the mouse MARCKS protein. The first 612 bp of the putative promoter was 89% identical to a corresponding region of the human promoter, and contained at least 59 potential transcription factor binding sites in analogous locations; both human and mouse promoters lacked TATA boxes. The mouse genomic probe was used to localize the mouse gene to chromosome 10, in the middle of a linkage group that corresponds to a region on human chromosome 6q. These data strongly suggested that the human gene would localize to 6q21. This was confirmed by studies of DNA from a patient with del(6)(q21), in which expression of the human gene encoding MARCKS, MACS, was only about 50% of normal; MARCKS mRNA expression in lymphoblast RNA from this patient was only 22% of normal. These studies confirm that the mouse and human MARCKS proteins are products of the same genes in their respective species; differences in their primary sequence can therefore be attributed to species variation rather than to the existence of related genes.

Characteristics of the F52 protein, a MARCKS homologue

A recently cloned mouse cDNA designated F52 encodes a putative protein with striking sequence similarity to the MARCKS protein, a major cellular substrate for protein kinase C (PKC). Major regions of sequence similarity include the amino-terminal myristoylation consensus sequence and the central calmodulin-binding/PKC phosphorylation site domain. The F52 protein was expressed in Escherichia coli with apparent M(r) 50,000; it was a substrate for PKC and comigrated on two-dimensional electrophoresis with a myristoylated protein whose phosphorylation was stimulated by phorbol 12-myristate 13-acetate in mouse neuroblastoma cells. The F52 protein also was myristoylated in E. coli by co-expression with N-myristoyltransferase. A 24-amino acid peptide derived from the protein's phosphorylation site domain was a good substrate for PKC; like the cognate MARCKS peptide, it was phosphorylated with high affinity (S0.5 = 173 nM) and positive cooperativity (KH = 5.4). The F52 peptide also bound calmodulin with high affinity (Kd = less than 3 nM); this binding could be disrupted by phosphorylation of the peptide with PKC, with a half-time of 8 min. The F52 protein is clearly a member of the MARCKS family as defined by primary sequence; in addition, the two proteins share several key attributes that may be functionally important.

Specific protein binding to a conserved region of the ornithine decarboxylase mRNA 5'-untranslated region

An RNA gel retardation assay was used to identify one or more cellular protein(s) (ornithine decarboxylase mRNA 5'-UTR binding protein (ODCBP)) that bind specifically to a conserved region of the 5'-untranslated region (5'-UTR) of rat ornithine decarboxylase (ODC) mRNA. Ultraviolet light cross-linking demonstrated that this protein has an apparent Mr = 58,000 in mammalian cells. Treatment with the oxidizing agent diamide prevented binding of the ODCBP to ODC mRNA; addition of beta-mercaptoethanol reversed this inhibition and permitted mRNA.ODCBP complex formation. Cytoplasmic extracts from a variety of animal cells and tissues demonstrated similar binding activities; however, there was marked tissue-specific expression of the protein in the rat, with brain, heart, lung, and testis containing large amounts, and kidney, spleen, and skeletal muscle expressing negligible amounts. Binding was completely prevented by several mutations within a highly conserved heptanucleotide region (CCAU/ACUC) that was within 61 bases of the initiation codon in ODC mRNAs from mammals, Xenopus, and Caenorhabditis elegans; mutations 5' and 3' of the conserved heptanucleotide domain had no effect on binding activity. Binding was not affected by manipulation of cellular polyamine levels or by treatment of cells with agents that stimulate ODC biosynthesis. Thus, we have identified a widely distributed cellular protein that binds to a conserved domain within the 5'-UTR of ODC mRNA from many animal species; functional consequences of this binding remain to be determined.

Evidence that extracellular signal-regulated kinases are the insulin-activated Raf-1 kinase kinases

The Raf-1 proto-oncogene protein kinase can be phosphorylated and activated after stimulation of cells with insulin and a variety of other growth factors and mitogens. We recently presented evidence that insulin and certain other growth factors activated one or more Raf-1 kinase kinase activities (Lee, R.M., Rapp, U. R., and Blackshear, P.J. (1991) J. Biol. Chem. 266, 10351-10357). In the present study, four peaks of Raf-1 kinase kinase activity were identified after anion-exchange chromatography of cell lysates, and two of these were activated by insulin. Further chromatographic characterization of these two peaks of insulin-activated kinase activity indicated that they contained three apparently distinct kinase activities. Two of these activities comigrated with immunoreactive extracellular signal-regulated kinases (ERK) 1 and 2 (mitogen-activated protein kinase) through three different chromatographic separations. Both ERK1 and ERK2 phosphorylated Raf-1 with reasonably high affinity (Km for ERK1 = 90 nM; Km for ERK2 = 120 nM), and produced similar, complex phosphopeptide maps; both kinases also phosphorylated myelin basic protein. The third kinase activity also phosphorylated Raf-1 and myelin basic protein but did not comigrate exactly with either immunoreactive ERK1 or ERK2. We conclude that two and possibly three insulin-activated Raf-1 kinase kinases are members of the ERK family.

Protein kinase C substrate and inhibitor characteristics of peptides derived from the myristoylated alanine-rich C kinase substrate (MARCKS) protein phosphorylation site domain

The phosphorylation sites in the myristoylated alanine-rich C kinase substrate or MARCKS protein consist of four serines contained within a conserved, basic region of 25 amino acids, termed the phosphorylation site domain. A synthetic peptide comprising this domain was phosphorylated by both protein kinase C and its catalytic fragment with high affinity and apparent positive cooperativity. Tryptic phosphopeptides derived from the peptide appeared similar to phosphopeptides derived from the phosphorylated intact protein. The peptide was phosphorylated by cAMP- and cGMP-dependent protein kinases with markedly lower affinities. In peptides containing only one of the four serines, with the other three serines replaced by alanine, the affinities for protein kinase C ranged from 25 to 60 nM with Hill constants between 1.8 and 3.0. The potential pseudosubstrate peptide, in which all four serines were replaced by alanines, inhibited protein kinase C phosphorylation of histone or a peptide substrate with an IC50 of 100-200 nM with apparently non-competitive kinetics; it also inhibited the catalytic fragment of protein kinase C with a Ki of 20 nM, with kinetics of the mixed type. The peptide did not significantly inhibit the cAMP- and cGMP-dependent protein kinases. It inhibited Ca2+/calmodulin-dependent protein kinases I, II, and III by competing with the kinases for calmodulin. In addition, the peptide inhibited the Ca2+/calmodulin-independent activity of a proteolytic fragment of Ca2+/calmodulin protein kinase II, with an IC50 approximately 5 microM. Thus, the phosphorylation site domain peptide of the MARCKS protein is a high affinity substrate for protein kinase C in vitro; the cognate peptide containing no serines is a potent but not completely specific inhibitor of both protein kinase C and its catalytic fragment.

The human myristoylated alanine-rich C kinase substrate (MARCKS) gene (MACS). Analysis of its gene product, promoter, and chromosomal localization

The expression of a major cellular substrate for protein kinase C, the MARCKS protein, is regulated in a cell-, tissue-, and developmental stage-specific fashion; in addition, this expression can be stimulated acutely by various cytokines in certain cell types. We have begun to characterize the human gene in order to elucidate the genetic elements responsible for this highly regulated expression. We first cloned a human MARCKS cDNA, which encoded a predicted protein of 332 amino acids (Mr 31,600) that was approximately 89, 74, and 59% identical to the bovine, mouse, and chicken proteins, respectively. Regions conserved at the amino acid level included the amino-terminal myristoylation consensus sequence, the site of intron splicing, and the phosphorylation site domain. The human cDNA was used to demonstrate that tumor necrosis factor-alpha could rapidly stimulate MARCKS gene transcription in the human promyelocytic leukemia cell line HL60. Genomic clones were then isolated; sequence analysis identified a putative promoter region that had no TATA box and contained multiple transcription initiation sites in a region spanning 57 base pairs (bp). This was followed by a 5'-untranslated region of approximately 400 bp, which displayed a complex predicted secondary structure with a delta G of -73.4 kcal/mol. Plasmid constructions containing between 52 and 1453 bp of the human MARCKS promoter linked to the human growth hormone gene were then used in transient expression experiments. Constructions containing 52 and 110 bp of the MARCKS promoter did not exhibit promoter function while the larger constructions all exhibited promotor function; the 248-bp fragment of the MARCKS promoter was 80% as effective as the human ferritin promoter in stimulating expression of human growth hormone in intact cells. Using an insert from the human genomic clone as a probe, we identified human chromosome 6, q21-qter, as the location of the MARCKS gene; this has been assigned the gene symbol MACS.

Insulin activation of protein kinase C: a reassessment

Although insulin is known to activate several protein serine/threonine protein kinases, its ability to activate protein kinase C remains controversial. We reinvestigated this question, taking advantage of several technical advances such as the development of fibroblast cell lines that overexpress normal human insulin receptors, and the development of antibodies to and expression vectors for the myristoylated, alanine-rich C kinase substrate (MARCKS) protein, a major cellular substrate for protein kinase C. In HIR 3.5 cells, a mouse 3T3 cell derivative that expresses about 6 x 10(6) human insulin receptors/cell, insulin (70 nM for 10 min) stimulated phosphorylation of the MARCKS protein by approximately 2-fold (p less than 0.005). This phosphorylation was not further increased by different times of insulin exposure, different insulin concentrations, or longer periods of serum deprivation. The insulin stimulation represented about 14% of the response to phorbol 12-myristate 13-acetate and about 17% of the response to 10% fetal calf serum. No significant stimulation of MARCKS protein phosphorylation was seen in four other insulin-sensitive cell lines, in which insulin is known to activate other protein serine/threonine kinases: HIRC-B, BC3H-1, 3T3-L1 adipocytes, and H35 rat hepatoma cells made to stably express the MARCKS protein. In these four cell lines, serum and/or phorbol 12-myristate 13-acetate exerted a large stimulatory effect on MARCKS protein phosphorylation. We conclude that insulin may activate protein kinase C to a minor extent in certain cell types that vastly overexpress insulin receptors; however, we believe that this effect of insulin is unlikely to be of physiological importance.

Evidence for one or more Raf-1 kinase kinase(s) activated by insulin and polypeptide growth factors

The protein product of the Raf-1 proto-oncogene is a protein serine/threonine kinase that is activated after stimulation of cells with insulin and other mitogens. To investigate the mechanism of this activation, we used purified Raf-1 expressed in E. coli as a substrate for a putative Raf-1 protein kinase kinase. In three different insulin-sensitive cell types, insulin activated Raf-1 kinase kinase activity in crude cytosolic cellular fractions. The insulin stimulation of this activity was evident as early as 2 min after exposure to insulin, maximal at 5-8 min, and inapparent at 15 min. Phosphoamino acid analysis of phosphorylated Raf-1 revealed that serine was the primary phosphate acceptor for the insulin-activated kinase or kinases; small amounts of phosphothreonine were also detected. The insulin effect occurred in cells depleted of protein kinase C, and in extracts depleted of endogenous Raf-1 kinase by immunodepletion; these data argue against protein kinase C or Raf-1 kinase itself being the insulin-stimulated activity. The insulin-activated kinase or kinases phosphorylated the Raf-1 protein on multiple sites in vitro, as evidenced by tryptic mapping; at least some of these appeared to overlap with sites phosphorylated in response to serum in intact cells. Several other mitogens and growth factors stimulated Raf-1 kinase kinase activity, including epidermal growth factor, platelet-derived growth factor, fibroblast growth factor, serum, and phorbol 12-myristate 13-acetate. This insulin- and mitogen-stimulated Raf-1 kinase kinase activity may play a role in mediating the phosphorylation and possibly the activation of the Raf-1 kinase by insulin and other growth factors.

Epidermal growth factor and other mitogens induce binding of a protein complex to the c-fos serum response element in human astrocytoma and other cells

Rapid stimulation of c-fos transcription by many agonists requires the serum response element (SRE), which binds at least two distinct nuclear proteins, p67SRF and p62TCF. Using nuclear protein extracts from 1321-N1 human astrocytoma cells, we investigated ligand-induced changes in binding of these proteins to SRE probes. In these cells c-fos mRNA expression can be induced by epidermal growth factor (EGF) through protein kinase C-independent pathways and by phorbol esters through protein kinase C. We detected two DNA-protein complexes that formed specifically with the SRE (bands 1 and 2). Band 2 formation was increased 4-6 min after stimulation with EGF as well as serum and phorbol esters; this peaked at 10-30 min and returned to basal levels by 60 min. Induction of band 2 formation preceded the onset and peak accumulation of c-fos mRNA (15 and 30 min after EGF stimulation, respectively) and its return to basal levels (by 1-2 h). Band 2 formation was also increased A431 cells stimulated with EGF and in HeLa and Swiss-3T3 cells stimulated with serum. We found that band 1 contained p67SRF bound to the SRE; band 2 contained p67SRF and a second protein. Gel shift analyses using [35S]methionine-labeled p67SRF and nonradioactive DNA probes suggested that hormone treatment most likely modified the second protein component of band 2. Transient transfection of 1321-N1 cells with plasmids containing point mutations that prevented band 2 formation in vitro also abolished induction of c-fos transcription in vivo as assayed by RNase protection analysis. Thus, hormone-stimulated formation of the protein-DNA complex represented by band 2 may be involved in the activation of c-fos transcription.

Phosphorylation-dependent binding of a synthetic MARCKS peptide to calmodulin

A 25-amino acid peptide, containing the four protein kinase C (PKC) phosphorylation sites and the calmodulin (CaM) binding domain of the myristoylated alanine-rich C kinase substrate (MARCKS) protein, has been synthesized and used to determine the effects of phosphorylation on its binding and regulation of CaM. PKC phosphorylation of this peptide (3.0 mol of Pi/mol of peptide) produced a 200-fold decrease in its affinity for CaM. PKC phosphorylation of the peptide resulted in its dissociation from CaM over a time course that paralleled the phosphorylation of 1 mol of serine/mol of peptide. The peptide inhibited CaM's binding to myosin light chain kinase and CaM's stimulation of phosphodiesterase and calcineurin. PKC phosphorylation of the peptide resulted in a rapid release of bound CaM, allowing its subsequent binding to myosin light chain kinase (t1/2 = 1.6 min), stimulation of phosphodiesterase (t1/2 = 1.2 min) and calcineurin (t1/2 = 1.7 min). Partially purified MARCKS protein produced a similar inhibition of CaM-phosphodiesterase which was reversed by PKC phosphorylation. PKC phosphorylation of the peptide occurred primarily at serine 8 and serine 12, and phosphorylation of serine 12 regulated peptide affinity for CaM. Thus, PKC phosphorylation of the peptide and the MARCKS protein results in the rapid release of CaM and the subsequent activation of CaM-dependent enzymes. This process might allow for interplay between PKC and CaM-dependent signal transduction pathways.

Insulin induction of ornithine decarboxylase. Importance of mRNA secondary structure and phosphorylation of eucaryotic initiation factors eIF-4B and eIF-4E

We investigated the possibility that insulin could stimulate translation of ornithine decarboxylase (ODC) mRNA in a murine fibroblast cell line that expresses large numbers of human insulin receptors (HIR 3.5 cells). Within 3 h after exposure to 70 nM insulin, ODC enzyme activity increased approximately 50-fold and mRNA accumulation 3-fold in the HIR 3.5 cells but not in normal fibroblasts. Pretreatment of cells with cycloheximide completely inhibited insulin-stimulated ODC expression; actinomycin D partially inhibited this effect. To determine the influence of the 5' untranslated region (5'UTR) of ODC mRNA on insulin-regulated ODC expression, plasmids were constructed which contained sequences from the 5'UTR of a rat ODC mRNA interposed between the ferritin promoter and the coding region of the human growth hormone gene. These constructions were then expressed transiently in HIR 3.5 cells. Insulin stimulated a 2-4-fold change in growth hormone accumulation in the medium of cells transiently expressing plasmids containing the entire 5'UTR of ODC mRNA or just the 5'-most 115 bases, a G/C-rich conserved sequence predicted to form a stem-loop structure and shown previously to be responsible for constitutive inhibition of translation. There was a direct correlation between the extent of insulin stimulation and the predicted secondary structure of the added 5'UTR sequences. To determine whether this effect might be due to insulin activation of initiation factors responsible for melting mRNA secondary structure, we examined the effect of insulin on the phosphorylation states of two such factors, eucaryotic initiation factors eIF-4B and eIF-4E. Insulin stimulated the phosphorylation of both initiation factors; this stimulation was evident at 15 min and maximal by 60 min. These results suggest a potential general mechanism by which insulin could preferentially stimulate translation of mRNAs whose 5'UTRs exhibit significant secondary structure by activating initiation factors involved in melting such secondary structures.

Cellular expression of mutant insulin receptors interferes with the rapid transcriptional response to both insulin and insulin-like growth factor I

We examined the expression of the proto-oncogene c-fos and the early growth response gene, Egr-1, in Rat 1 fibroblasts expressing high levels of normal or mutated human insulin receptors (McClain, D. A., Maegawa, H., Lee, J., Dull, T. J., Ullrich, A., and Olefsky, J. M. (1987) J. Biol. Chem. 262, 14663-14671). In cells expressing large numbers of normal human insulin receptors (HIRc-B cells), insulin (greater than or equal to 0.7 nM) stimulated the rapid accumulation of mRNAs for both genes. This response was blunted, but not lost, in cells expressing large numbers of human insulin receptors missing 43 amino acids at the carboxyl terminus of the beta-subunit. In contrast, the insulin response was completely absent in cells expressing large numbers of receptors that contained a mutation at the ATP-binding site that destroyed intrinsic protein tyrosine kinase activity (A/K 1018-B cells). This mutation also suppressed the modest transcriptional response to insulin that occurred in the parental Rat 1 cells. The transcriptional response to serum was normal in the A/K 1018-B cells, even after protein kinase C depletion; however, the response to insulin-like growth factor I was essentially lost. These studies suggest that overexpression of a kinase-deficient insulin receptor can suppress the transcriptional response to both insulin and insulin-like growth factor I that is ordinarily transduced through endogenous insulin and insulin-like growth factor I receptors, respectively. Competition for shared substrates of these related receptor kinases is a potential mechanism for this effect.

Neuropeptide Y stimulation of myosin light chain phosphorylation in cultured aortic smooth muscle cells

Neuropeptide Y (NPY) is released from an extensive network of postganglionic sympathetic perivascular neurons. NPY has been shown to affect vascular tone postsynaptically by 1) directly stimulating contraction; 2) inhibiting vasorelaxation; and 3) potentiating contraction elicited by exogenous vasoconstrictors. The molecular mechanisms mediating these effects of NPY are undefined. Therefore, we examined the possibility that NPY could stimulate smooth muscle contraction through myosin light chain phosphorylation in cultured porcine aortic smooth muscle cells. NPY (100 nM) caused a rapid, transient increase in myosin light chain (MLC) phosphorylation, an important regulatory event in the initiation of smooth muscle contraction. NPY-stimulated MLC phosphorylation was prevented by preincubation of cells with pertussis toxin and was independent of extracellular Ca2+. In parallel studies, NPY alone had no detectable effect on cellular cAMP or cGMP content; however, NPY potently inhibited forskolin-stimulated cAMP accumulation (IC50 = 0.03 nM) through a pertussis toxin-sensitive pathway. NPY had no detectable effect on basal phosphoinositide hydrolysis or protein kinase C activation but enhanced angiotensin II-stimulated production of inositol phosphates and activation of protein kinase C. These results indicate that NPY-stimulated MLC phosphorylation can occur in the absence of detectable changes in cAMP content, cGMP content, inositol phosphate production, or protein kinase C activation; however, the interactions between NPY and other vasoactive agents may be mediated by the indirect effects of NPY on adenylate cyclase activity and phosphoinositide hydrolysis.

Rapid insulin-stimulated accumulation of an mRNA encoding a proline-rich protein

By differential hybridization screening of a cDNA library derived from insulin-stimulated cells, we selected a clone which hybridized to an mRNA species that rapidly accumulated in response to insulin. The insert from this clone encoded a putative polypeptide of Mr 33,600, pI 11.2; because the protein was enriched in proline residues (14.4 mol %) and contained three Pro-Pro-Pro-Pro repeats, we have tentatively labeled it tris-tetraprolin (TTP). The function of this protein is not known, but it contains two regions very rich in proline (30-40 mol %); similar proline-rich regions have been shown to be involved in transcriptional activation by other proteins. The mRNA (2.0 kilobases) encoding the TTP protein was essentially undetectable in serum-deprived HIR 3.5 cells, but accumulated dramatically within 10 min of stimulation by insulin. This effect appeared to be due to insulin acting through the intrinsic protein-tyrosine kinase activity of its own receptor. Insulin induction of TTP mRNA accumulation was prevented by actinomycin D and superinduced by cycloheximide. Accumulation of TTP mRNA was also stimulated by a variety of growth factors and active phorbol esters; however, the insulin effect was virtually normal in cells depleted of protein kinase C. A single TTP gene appeared to be present in the mouse genome. This gene joins the group of genes whose members are rapidly transcribed in response to insulin and other mitogens.

Insulin activates the Raf-1 protein kinase

Several growth factors and mitogens have been shown to activate the proto-oncogene product Raf-1 protein kinase in murine fibroblasts, apparently through a direct agonist-stimulated tyrosine phosphorylation of the Raf-1 protein. We investigated the possibility that insulin could also activate the Raf-1 kinase, since its receptor also contains an intrinsic insulin-activated protein tyrosine kinase activity. In several cell lines expressing relatively large numbers of insulin receptors, insulin rapidly stimulated the phosphorylation of immunoreactive Raf-1 protein. In H35 cells, a line of well differentiated rat hepatoma cells, the effect of insulin was maximal by 6 min and at 7 nM insulin and occurred normally in cells virtually completely depleted of protein kinase C activity. The insulin-stimulated increase in Raf-1 protein phosphorylation occurred concurrently with a 3-fold increase in Raf-1 protein kinase activity. However, phosphoamino acid analysis showed that only phosphoserine and a trace of phosphothreonine were present in the Raf-1 protein after insulin stimulation of the cells. This was true even when investigated at shorter times (4 min) after insulin stimulation and despite the use of phosphotyrosine phosphatase inhibitors. We conclude that insulin can rapidly activate the Raf-1 kinase in some insulin-sensitive cell types but that this activation probably occurs through a mechanism distinct from direct phosphorylation of the Raf-1 protein by the insulin receptor protein tyrosine kinase.

Regulation of rat ornithine decarboxylase mRNA translation by its 5'-untranslated region

Ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis, is a highly inducible protein whose expression involves a complex and variable array of regulatory mechanisms. We investigated the influence of the 5'-untranslated region (5'UTR) of the rat ODC mRNA on translation of the mRNA in a cell-free system and in cultured mammalian cells. ODC mRNA containing the full-length 5'UTR was translated in reticulocyte lysates at approximately 5% of the rate of mRNA containing no ODC 5' leader sequences. The complete 5'UTR inhibited expression of a heterologous gene product, human growth hormone, to the same extent in cultured mammalian cells. Furthermore, the 5'-most 130 bases of the rat ODC 5'UTR, a conserved G/C-rich region predicted to form a stable stem-loop structure (delta G = -68 kcal/mol), repressed translation to the same extent as the entire 5'UTR, both in the lysates and in intact cells. The 3'-most 160 bases of the 5'UTR, containing a small upstream open reading frame, decreased expression by 50-65% both in vitro and in intact cells, compared with controls lacking any ODC 5'UTR sequences. Mutation of the initiation codon AUG beginning this upstream open reading frame to GCG restored expression to rates equivalent to those seen in constructions containing no ODC 5'UTR sequences. We conclude that the rat ODC mRNA 5'UTR can inhibit translation of ODC mRNA both in vitro and in vivo, and that the predicted stem-loop structure at the 5' end of the 5'UTR is both necessary and sufficient for this inhibition.

Neuropeptide Y binding and inhibition of cAMP accumulation in human neuroepithelioma cells

The specific binding of 125I-labeled neuropeptide Y (NPY) and the biological response to NPY receptor activation were measured in cultured human neuroepithelioma (SK-N-MC) cells. A single class of high-affinity binding sites [dissociation constant (KD) = 0.2 nM] was characterized both by equilibrium binding of 125I-NPY concentrations less than 1 nM and kinetically by the initial rates of 125I-NPY association and dissociation. Specific 125I-NPY binding was decreased in a concentration-dependent manner by inclusion of guanine nucleotides in the incubation medium. The existence of multiple affinity states or NPY receptor subtypes was suggested by 1) a Hill coefficient of less than 1.0 obtained when analyzing equilibrium binding with 125I-NPY concentrations greater than 1 nM, 2) biphasic dissociation of 125I-NPY, 3) an increase in the component of rapid dissociation and decrease in the component of slow dissociation when guanine nucleotides were present during dissociation of 125I-NPY, and 4) displacement of 125I-NPY by unlabeled peptide with a slope factor of 0.6. Exposure of intact cells to NPY caused a concentration-dependent pertussis toxin-sensitive inhibition of forskolin-stimulated cellular adenosine 3',5'-cyclic monophosphate (cAMP) accumulation [50% effective concentration (EC50) = 0.4 nM]. In contrast, NPY had no effect on cellular inositol phosphate content or protein kinase C activation. These results demonstrate that NPY binds specifically to a G protein-linked receptor that inhibits adenylate cyclase in SK-N-MC cells.

Giant two-dimensional gel electrophoresis: methodological update and comparison with intermediate-format gel systems

Two-dimensional (2-D) gel electrophoresis methods for separating complex mixtures of proteins have not changed fundamentally since their original description in the late 1970's. Nevertheless, 2-D gel resolution has improved substantially as a result of a series of incremental modifications. One of these was the development of the "giant-gel" format, using gels measuring at least 30 x 30 cm to provide the highest resolution 2-D gel system available. As originally described, this procedure has several important limitations: it requires custom-built equipment; it is expensive in terms of time, reagents, film and support matrices; and it generates gels which are difficult to manupulate, particularly for silver staining. This report describes modifications in the giant gel procedure to permit use of a commercially available gel apparatus and to obtain giant gels of improved mechanical strength suitable for silver staining. The resolution of giant gels is compared with that obtained using two systems currently being marketed for use by laboratories performing large numbers of 2-D gel analyses. The smaller format gels resolved fewer proteins, by 30-40%, compared with the giant gels. This difference in resolving power suggests that giant gels will continue to be useful in selected applications.

Phosphorylation-regulated calmodulin binding to a prominent cellular substrate for protein kinase C

We have evaluated the possibility that a major, abundant cellular substrate for protein kinase C might be a calmodulin-binding protein. We have recently labeled this protein, which migrates on sodium dodecyl sulfate-gel electrophoresis with an apparent Mr of 60,000 from chicken and 80,000-87,000 from bovine cells and tissues, the myristoylated alanine-rich C kinase substrate (MARCKS). The MARCKS proteins from both species could be cross-linked to 125I-calmodulin in a Ca2+-dependent manner. Phosphorylation of either protein by protein kinase C prevented 125I-calmodulin binding and cross-linking, suggesting that the calmodulin-binding domain might be located at or near the sites of protein kinase C phosphorylation. Both bovine and chicken MARCKS proteins contain an identical 25-amino acid domain that contains all 4 of the serine residues phosphorylated by protein kinase C in vitro. In addition, this domain is similar in sequence and structure to previously described calmodulin-binding domains. A synthetic peptide corresponding to this domain inhibited calmodulin binding to the MARCKS protein and also could be cross-linked to 125I-calmodulin in a calcium-dependent manner. In addition, protein kinase C-dependent phosphorylation of the synthetic peptide inhibited its binding and cross-linking to 125I-calmodulin. The peptide bound to fluorescently labeled 5-dimethylaminonaphthalene-1-sulfonyl-calmodulin with a dissociation constant of 2.8 nM, and inhibited the calmodulin-dependent activation of cyclic nucleotide phosphodiesterase with an IC50 of 4.8 nM. Thus, the peptide mimics the calmodulin-binding properties of the MARCKS protein and probably represents its calmodulin-binding domain. Phosphorylation of these abundant, high affinity calmodulin-binding proteins by protein kinase C in intact cells could cause displacement of bound calmodulin, perhaps leading to activation of Ca2+-calmodulin-dependent processes.

Insulin-stimulated protein tyrosine phosphorylation in intact cells evaluated by giant two-dimensional gel electrophoresis

We have studied the insulin-stimulated phosphorylation of proteins in NIH 3T3 cells expressing high numbers of human insulin receptors (HIR 3.5 cells) using the technique of giant two-dimensional gel electrophoresis. In serum-deprived cells, insulin stimulated the phosphorylation of more than 25 proteins; all but two of these were also phosphorylated in response to 15% (v/v) fetal bovine serum, which also stimulated the phosphorylation of additional proteins thought to be direct substrates for protein kinase C. In cells pretreated insulin specifically stimulated the phosphorylation insulin specifically stimulated the phosphorylation of at least 26 predominantly cytosolic proteins, only one of which was observed in insulin-treated cells not exposed to phenylarsine oxide. Serum was without effect in cells pretreated with phenylarsine oxide. In phenylarsine oxide-pretreated cells, phosphoamino acid analysis of 10 of the most highly labeled insulin-stimulated phosphoproteins showed that all 10 were labeled predominantly or exclusively on tyrosine residues. The phosphorylation of several of these could be stimulated in vitro by the addition of insulin to a detergent extract of cells in the presence of Mn2+ and ATP. In general, the insulin-stimulated phosphorylations observed in the presence of phenylarsine oxide were more rapid than those observed in its absence. Finally, a variety of other growth factors and mitogens did not stimulate any of the insulin-stimulated phosphorylations in the presence of phenylarsine oxide. Thus, the use of this inhibitor apparently unmasked a number of novel insulin-specific protein phosphorylations that were ordinarily undetectable. We suggest that at least some of these proteins may be direct substrates for the insulin receptor protein tyrosine kinase and may play significant roles in insulin action.

Molecular cloning, sequence, and expression of a cDNA encoding the chicken myristoylated alanine-rich C kinase substrate (MARCKS)

Little is known about the important cellular substrates for protein kinase C (PKC) and their function in the cellular processes influenced by this kinase. This paper describes the molecular characteristics of a prominent cellular substrate for PKC in chicken cells, known as the myristoylated alanine-rich C kinase substrate, or MARCKS protein. The chicken protein was studied because it was apparently at least 20 kilodalton smaller than its mammalian counterpart; we hoped that regions of sequence similarity might point to conserved regions of biological importance. Using the bovine MARCKS cDNA as a probe, we selected a positive clone from a chicken brain cDNA library that contained an insert of about 1.5 kilobase, in which a single open reading frame encoded a protein of 281 amino acids, 27.7 kilodaltons, pI 5.26. This protein contained the sequences of ten tryptic peptides derived from the purified chicken brain protein. Expression of the cDNA insert in mammalian cells confirmed that the open reading frame encoded a protein that comigrated on two-dimensional electrophoresis with the authentic chicken protein, and could be phosphorylated by exposure of the cells to active phorbol esters. When the chicken and bovine protein sequences were compared, the two major regions of sequence identity were: 1) the amino terminal region containing a myristoylation consensus sequence and an mRNA splice site, and 2) a highly basic internal domain of 25 amino acids that contained all of the serines known to be phosphorylated by PKC in the intact protein. These conserved regions are likely to represent domains of some functional importance for this widely distributed cellular substrate for PKC.

Myristoylated and nonmyristoylated forms of a protein are phosphorylated by protein kinase C

Activation of protein kinase C is thought to require association of the kinase with the cell membrane. It has been assumed that cellular substrates for the kinase must likewise be associated with membranes, and previous studies with membrane-associated myristoylated proteins have supported this view. It is now shown that a mutation that prevents the normal amino-terminal myristoylation of a prominent cellular substrate of protein kinase C, and appears to prevent its membrane association, does not prevent the normal phosphorylation of this protein in intact cells in response to phorbol esters. Thus, membrane association may not be required in order for protein kinase C substrates to undergo phosphorylation.

Insulin rapidly induces the biosynthesis of elongation factor 2

Insulin increases the rate of overall protein synthesis in many cells and tissues, while inducing the preferential expression of individual proteins. To identify and characterize such proteins, NIH 3T3 cells stably expressing more than 10(6) human insulin receptors per cell (HIR 3.5; Whittaker, J., Okamoto, A. K., Thys, R., Bell, G. I., Steiner, D. F., and Hofmann, C. A. (1987) Proc. Natl. Acad. Sci. U. S. A. 84, 5237-5241) were treated with insulin in the presence of [35S]methionine, and labeled proteins were separated using ultra-high resolution ("giant") two-dimensional gel electrophoresis. Overall protein synthesis was enhanced as much as 3-fold by insulin treatment; the synthesis of approximately 1% of the 2,500 proteins visible on the gel autoradiographs was further selectively increased. By using immunoblotting, immunoprecipitation and comigration assays, we identified one of the rapidly induced proteins (Mr 96,000; pI 6.8) as eukaryotic elongation factor 2 (EF-2), a major component of the protein translation apparatus. Insulin induced the synthesis of EF-2 within 20 min of treatment, with a half-maximal dose of 10(-11) M. It was synthesized as a precursor form that was processed to a more basic mature species within 30 min. Long term treatment with insulin led to accumulation of EF-2 within the cell and prevented the substantial decrease in EF-2 concentration that occurred during serum deprivation. Finally, we found that insulin induction of EF-2 occurred normally in the presence of the RNA-transcription inhibitor, actinomycin D. Thus, insulin rapidly induced the synthesis of EF-2 predominantly or exclusively at the level of mRNA translation.

Characterization of the phosphorylation sites in the chicken and bovine myristoylated alanine-rich C kinase substrate protein, a prominent cellular substrate for protein kinase C

Little is known about the important cellular substrates for protein kinase C and their potential roles in mediating protein kinase C-dependent processes. We evaluated the protein kinase C phosphorylation sites in a major cellular substrate for the kinase, a protein of apparent Mr 80,000 in bovine and 60,000 in chicken tissues; we have recently determined the primary sequences of these proteins and tentatively named them the myristoylated alanine-rich C kinase substrates. The proteins were purified to apparent homogeneity from bovine and chicken brains, phosphorylated with protein kinase C, digested with trypsin, and the phosphopeptides purified and sequenced. Four distinct phosphopeptides were identified from both the bovine and chicken proteins. Two of the phosphorylated serines were contained in the repeated motif FSFKK, one in the sequence LSGF, and one in the sequence SFK. All four sites were contained within a basic domain of 25 amino acids which was identical in the chicken and bovine proteins. All of the sites phosphorylated in the cell-free system appeared to be phosphorylated in intact cells; an additional site may have been present in the proteins from intact cells. The identity of the phosphorylation site domains from two proteins of overall 65% amino acid sequence identity suggests a potential role for this domain in the physiological function of the myristoylated alanine-rich C kinase substrate proteins.

Basal-rate intravenous insulin infusion compared to conventional insulin treatment in patients with type II diabetes. A prospective crossover trial

We compared continuous basal-rate intravenous insulin infusion, delivered by means of a totally implantable pump, to two types of conventional insulin administration in patients with type II (non-insulin-dependent) diabetes in a prospective crossover trial. Ten patients entered the study, and 5 completed all three 8-mo study periods. When results from the infusion study period were compared with results from the period involving single daily injections of ultralente insulin, significant improvements were noted in the pump arm in glycosylated hemoglobin concentrations (which were nearly normal), M-component values, mean daily outpatient fasting blood glucose concentrations, mean fasting and 24-h blood glucose concentrations during an inpatient 24-h glycemic profile, and urinary glucose concentrations. When the pump arm was compared to a period of single daily injections of lente insulin, three of six monthly mean fasting blood glucose concentrations and overall means for the entire study period were significantly lower during the pump arm than during the lente arm; in addition, significantly fewer hypoglycemic reactions were noted during infusion therapy than during lente therapy. Finally, mealtime free-insulin and C-peptide excursions appeared to be greater during infusion treatment when compared with lente or ultralente treatment. In the 50% of patients who completed the study, it appeared that significant improvements in glycemic control could be achieved by simple basal-rate intravenous insulin infusion compared with conventional treatment with single daily injections of ultralente or lente insulin without an increased incidence of symptomatic hypoglycemia.

Molecular cloning, characterization, and expression of a cDNA encoding the "80- to 87-kDa" myristoylated alanine-rich C kinase substrate: a major cellular substrate for protein kinase C

We isolated and sequenced a cDNA clone encoding the bovine "80- to 87-kDa" protein, a major cellular substrate for protein kinase C. An open reading frame of 1005 base pairs predicted a protein of 335 amino acids (Mr, 31,949). Despite this predicted size, the protein migrated on SDS/polyacrylamide gels with an apparent molecular weight of 80-87,000 after expression of the cDNA in cells lacking the protein. It was highly enriched in alanine (28.4 mol %), contained an amino-terminal myristoylation consensus sequence, and included a 25-residue basic domain containing the known protein kinase C phosphorylation sites. Two mRNA species (2.6 and 4.4 kilobases) were most highly expressed in brain, spinal cord, spleen, and lung, in parallel with the distribution of immuno-reactive protein. Genomic blot analysis indicated the likelihood of a single gene coding for this mRNA. We propose the name myristoylated alanine-rich C kinase substrate (MARCKS) for this protein.

Rat ornithine decarboxylase gene. Nucleotide sequence, potential regulatory elements, and comparison to the mouse gene

Expression of ornithine decarboxylase is regulated by a variety of hormonal and other stimuli in rat cells and tissues. To study this phenomenon at the molecular level, we isolated and sequenced a cDNA-encoding rat ornithine decarboxylase and deduced its amino acid sequence. The cDNA clone was used to isolate a clone from a rat genomic library which contained the sequence of the entire rat ornithine decarboxylase gene. The gene comprised 12 exons and 11 introns and spanned 7.7 kilobases. Two polyadenylation signals (AATAAA) were located 310 and 697 base pairs 3' to the translational termination codon and were responsible for the occurrence of two hybridizing mRNA species in Northern blots of rat cells and tissues. S1 nuclease mapping suggested that there were multiple transcriptional start sites; the major one appeared to be located 2269 base pairs of genomic sequence 5' to the ATG translational initiation site, representing 274 bases of mRNA. Several potential regulatory elements were identified in the 5'-promoter regions or in the first intron: a TATA box, GC boxes, AP-1 and AP-2 binding sites, a cAMP-responsive element, a glucocorticoid regulatory element, and RNA polymerase III promoter sequences. The 5'-noncoding region of the mRNA was extremely rich in G + C; secondary structure predictions suggested that almost this entire region could form stable secondary structures, with an overall free energy of formation (delta G) of -114 kcal/mol. The potential regulatory elements identified in both the promoter region of the gene and the 5'-untranslated region of the mRNA may be involved in the complex regulation of rat ornithine decarboxylase expression.

Evidence against insulin-stimulated phosphorylation of calmodulin in 3T3-L1 adipocytes

To examine the possibility that insulin might stimulate calmodulin phosphorylation in intact cells, we compared autoradiographs of two-dimensional gels of [35S]methionine- and 32P-labeled proteins from 3T3-L1 adipocytes, before and after immunoprecipitation with anti-calmodulin antiserum. Insulin stimulated the phosphorylation of one or two proteins of approximately 22 kDa and pI 4.6; this increased phosphorylation was accompanied by an apparent shift in the position of the analogous [35S]methionine-labeled proteins towards the anode. In contrast, insulin had no effect on the phosphorylation state of another protein of 18-22 kDa and pI 4.6. This protein was very heavily labeled with [35S]methionine, co-migrated exactly with purified calmodulin, reacted specifically with two anti-calmodulin antibodies by Western blotting, and was specifically immunoprecipitated with the anti-calmodulin antiserum. Similar amounts of [35S]methionine-labeled calmodulin were immunoprecipitated from control and insulin-stimulated cells, arguing against the possibility that insulin-stimulated phosphorylation of calmodulin changed its affinity for the antibody. We conclude that calmodulin is phosphorylated to a negligible extent in serum-deprived 3T3-L1 adipocytes and that insulin does not stimulate its phosphorylation under conditions in which it stimulates the phosphorylation of one or more neighboring proteins.

High level, cell-specific expression of ornithine decarboxylase transcripts in rat genitourinary tissues

We evaluated transcript levels for the rate-limiting enzyme in polyamine biosynthesis, ornithine decarboxylase (ODC), in rat tissues by Northern blotting and in situ hybridization histochemistry, using a rat cDNA probe. ODC transcripts were expressed at a high level, relative to levels in other tissues, in the kidney and testis of the adult rat; maximal levels of transcripts in these tissues occurred after sexual maturation had taken place, i.e. between 20 and 150 days of age. In situ hybridization histochemistry revealed high level expression in the kidney, testis, prostate, and seminal vesicles of the male rat; this high level expression was limited to certain cell types: kidney, S3 cells of the proximal convoluted tubule; prostate and seminal vesicles, glandular or luminal epithelial cells; and testis, early spermatogenic cells. High level expression of ODC mRNA disappeared from the prostate and seminal vesicle epithelial cells after castration and reappeared with testosterone treatment; in contrast, levels of kidney ODC mRNA were essentially unchanged by castration and were similar in male and female adult rats. We conclude that high level ODC mRNA expression occurs in specific cell types in the adult rat, where it appears to be regulated by both androgen-dependent and independent mechanisms.

Protein kinases 1988: a current perspective

This review focuses on several recent developments in the field of protein kinases. In the area of protein serine/threonine kinases, much has been learned recently about protein kinase C structure and function. Novel lipid mediators, both stimulatory and inhibitory, have been discovered, and kinase has been shown to be an increasingly large family of gene products. Heterogeneity of cellular localization and function has been documented. Calcium/calmodulin-dependent protein kinases are now believed to consist of at least five enzymes, which range from those with extreme substrate specificity such as phosphorylase kinase and myosin light-chain kinases to calcium calmodulin kinase II, with several known substrates. Several of these enzymes appear to be important in synaptic transmission and, for calcium/calmodulin kinase III, in the regulation of protein synthesis. Several new examples of pseudosubstrate prototopes as endogenous kinase inhibitors have been described, including regions intrinsic to kinase primary sequences, which could serve as constitutive inhibitors of enzyme activity. In the field of protein tyrosine kinases, new enzyme species are being discovered at a rapid rate. There are several well-documented examples of kinase autophosphorylation on tyrosine leading to stimulation of catalytic activity. For the growth factor receptors with intrinsic protein tyrosine kinase activity, it now seems clear that kinase catalytic activity is necessary for most hormone effects on cells, with the general exceptions of ligand binding and, possibly, receptor cycling. Finally, several groups have recently described a close association between protein tyrosine kinases and a phosphatidylinositol kinase activity, a link that might eventually explain some of the initial steps in signal transduction that occur after kinase activation.