The steroidogenic acute regulatory protein (StAR): a window into the complexities of intracellular cholesterol trafficking

Stimulation of steroid-producing cells of the gonads and adrenals with tropic hormone results in a marked increase in steroid hormone synthesis within minutes. The rate-limiting step in this acute steroidogenic response is the transport of cholesterol from the outer to the inner mitochondrial membrane, where the first committed step in steroid synthesis is performed by the side-chain cleavage enzyme system. This process of cholesterol translocation is blocked by inhibitors of protein synthesis, suggesting that the effect of trophic hormones, acting through the intermediacy of cAMP, most likely involves the de novo synthesis of a protein that is rapidly inactivated. The recently identified steroidogenic acute regulatory (StAR) protein appears to be the most likely candidate for the "labile" protein, based on the following observations: 1) Expression of StAR in COS-1 cells engineered to contain the cholesterol side-chain cleavage system substantially augments pregnenolone formation; 2) StAR protein is expressed almost exclusively in steroid-producing cells, except the trophoblast of the human placenta, and its presence is correlated with steroid hormone production; 3) StAR mRNA increases in response to cAMP; 4) StAR is a target for serine phosphorylation mediated by protein kinase A, a process that is essential for maximizing StAR activity; and 5) lack of functional StAR causes the autosomal recessive disease, congenital lipoid adrenal hyperplasia, characterized by markedly impaired gonadal and adrenal steroid hormone synthesis. Studies on the mechanism of action of StAR revealed that import into mitochondria is not essential to its steroidogenesis-enhancing activity and more likely represents a means of rapidly inactivating StAR. Truncation mutations and site-directed mutations established that the C-terminus of the StAR protein contains the functionally important domains. The demonstration of steroidogenic activity of recombinant StAR protein on isolated mitochondria from bovine corpus luteum using protein that lacks the mitochondrial targeting sequence confirmed that StAR import is not essential for its steroidogenic activity and suggested that StAR acts directly on the outer mitochondrial membrane in the absence of intermediary cytosolic factors. Evidence that StAR functions as a cholesterol transfer protein raises the possibility that StAR acts directly on lipids of the outer mitochondrial membrane, probably stimulating cholesterol desorption from the sterol-rich outer membrane and its movement to the relatively sterol-poor inner membrane.

Unveiling the mechanism of action and regulation of the steroidogenic acute regulatory protein

Stimulation of steroid-producing cells of the gonads and adrenals with trophic hormone (LH, and ACTH, respectively) produces a marked increase in steroid hormone synthesis within minutes. The rate-limiting step in this acute steroidogenic response is the transport of cholesterol from the outer to the inner mitochondrial membrane, where the first committed step in steroid synthesis is performed by the side-chain cleavage enzyme system (P450scc), resulting in the production of pregnenolone. This process of cholesterol translocation is blocked by inhibitors of protein synthesis (i.e. cycloheximide) indicating that the effect of trophic hormones, acting through the intermediacy of cAMP, most likely involves the de novo synthesis of a protein that is rapidly inactivated. The recently identified steroidogenic acute regulatory protein (StAR) appears to be the most likely candidate for the labile protein: (1) StAR is synthesized in response to cAMP and the StAR preprotein disappears rapidly in the presence of inhibitors of protein synthesis; (2) StAR has an N-terminal targeting sequence that directs the protein to the mitochondria; and (3) StAR protein is expressed almost exclusively in steroid-producing cells, its presence is correlated with steroid hormone production, and lack of functional StAR causes the autosomal recessive disease congenital lipoid adrenal hyperplasia (lipoid CAH), characterized by markedly impaired gonadal and adrenal steroid hormone synthesis. We have demonstrated that StAR is a target for serine phosphorylation mediated by protein kinase A (PKA), a process that is essential to maximizing StAR activity. StAR import by mitochondria is not essential to its steroidogenesis enhancing activity, and more likely, represents a means of rapidly inactivating StAR. Truncation mutations and site-directed mutations in StAR demonstrated that the C-terminus of the protein contains the functionally important domains. Further, we have demonstrated potent steroidogenic activity of recombinant StAR protein on isolated mitochondria from bovine corpus luteum using protein that lacks the mitochondrial targeting sequence. These observations confirm that StAR import is not essential for its steroidogenic activity and suggest that StAR acts directly on the outer mitochondrial membrane in the absence of intermediary cytosolic factors. More recently, we have found that StAR functions as a cholesterol transfer protein that does not require a protein receptor or co-factor, suggesting that StAR acts directly on lipids of the outer mitochondrial membrane to promote cholesterol translocation.

Steroidogenic acute regulatory protein (StAR) acts on the outside of mitochondria to stimulate steroidogenesis

Steroidogenic acute regulatory protein (StAR) facilitates delivery of cholesterol to the inner mitochondrial membranes. StAR is imported into mitochondria and processed to a mature form by cleavage of the N-terminal mitochondrial targeting sequence. We produced His-tagged (His-tag StAR) constructs lacking the N-terminal 62 amino acids that encode the mitochondrial targeting sequence and examined their steroidogenic activity in intact cells and on isolated mitochondria. His-tag StAR proteins stimulated pregnenolone synthesis to the same extent as wild-type StAR when expressed in COS-1 cells transfected with the cholesterol side-chain cleavage system. His-tag StAR was diffusely distributed in the cytoplasm of transfected COS-1 cells, whereas wild-type StAR was localized to mitochondria. There was no evidence at the light or electron microscope levels for selective localization of His-tag StAR protein to mitochondrial membranes. We established an assay system using mitochondria isolated from bovine corpora lutea and purified recombinant His-tag StAR proteins expressed in E. coli. Recombinant His-tag StAR stimulated pregnenolone production in a dose- and time-dependent manner, functioning at nanomolar concentrations. A point mutant of StAR (A218V) that causes lipoid congenital adrenal hyperplasia was incorporated into the His-tag protein. This mutant was steroidogenically inactive in COS-1 cells and on isolated mitochondria. Our observations conclusively document that StAR acts on the outside of mitochondria, independent of mitochondrial import.

The mechanism of action of steroidogenic acute regulatory protein (StAR). StAR acts on the outside of mitochondria to stimulate steroidogenesis

Steroidogenic acute regulatory protein (StAR) plays an essential role in steroidogenesis, facilitating delivery of cholesterol to cytochrome P450scc on the inner mitochondrial membrane. StAR is synthesized in the cytoplasm and is subsequently imported by mitochondria and processed to a mature form by cleavage of the NH2-terminal mitochondrial targeting sequence. To explore the mechanism of StAR action, we produced 6-histidine-tagged N-62 StAR (His-tag StAR) constructs lacking the NH2-terminal 62 amino acids that encode the mitochondrial targeting sequence and examined their steroidogenic activity in intact cells and on isolated mitochondria. His-tag StAR proteins stimulated pregnenolone synthesis to the same extent as wild-type StAR when expressed in COS-1 cells transfected with the cholesterol side-chain cleavage system. His-tag StAR was diffusely distributed in the cytoplasm of transfected COS-1 cells whereas wild-type StAR was localized to mitochondria. There was no evidence at the light or electron microscope levels for selective localization of His-tag StAR protein to mitochondrial membranes. In vitro import assays demonstrated that wild-type StAR preprotein was imported and processed to mature protein that was protected from subsequent trypsin treatment. In contrast, His-tag StAR was not imported and protein associated with mitochondria was sensitive to trypsin. Using metabolically labeled COS-1 cells transfected with wild-type or His-tag StAR constructs, we confirmed that wild-type StAR preprotein was imported and processed by mitochondria, whereas His-tag StAR remained largely cytosolic and unprocessed. To determine whether cytosolic factors are required for StAR action, we developed an assay system using washed mitochondria isolated from bovine corpora lutea and purified recombinant His-tag StAR proteins expressed in Escherichia coli. Recombinant His-tag StAR stimulated pregnenolone production in a dose- and time-dependent manner, functioning at nanomolar concentrations. A point mutant of StAR (A218V) that causes lipoid congenital adrenal hyperplasia was incorporated into the His-tag protein. This mutant was steroidogenically inactive in COS-1 cells and on isolated mitochondria. Our observations conclusively document that StAR acts on the outside of mitochondria, independent of mitochondrial import, and in the absence of cytosol. The ability to produce bioactive recombinant StAR protein paves the way for refined structural studies of StAR and StAR mutants.

Phosphorylation of steroidogenic acute regulatory protein (StAR) modulates its steroidogenic activity

Steroidogenic acute regulatory protein (StAR) plays a critical role in steroid hormone synthesis. StAR is thought to increase the delivery of cholesterol to the inner mitochondrial membrane where P450scc resides. Tropic hormones acting through the intermediacy of cAMP rapidly increase pregnenolone synthesis, and this rapid steroidogenic response is believed to be due to StAR's action. The StAR protein contains two consensus sequences for phosphorylation catalyzed by protein kinase A that are conserved across all species in which the amino acid sequence of the StAR protein has been determined. We demonstrated that human StAR expressed in COS-1 cells exists in at least four species detectable by two-dimensional gel electrophoresis followed by Western blotting. The two more acidic species disappeared after treatment of the cell extracts with alkaline phosphatase. 32P was incorporated into StAR protein immunoprecipitated from COS-1 cell extracts, and a 10-min treatment with 8-bromo-cAMP increased 32P incorporation into the StAR preprotein. StAR protein generated by in vitro transcription/translation was phosphorylated by the protein kinase A catalytic subunit in the presence of [gamma-32P]ATP. Mutation of potential sites for protein kinase A-mediated phosphorylation at serine 57 and serine 195 to alanines, individually, reduced 32P incorporation from labeled ATP into StAR preprotein produced by in vitro transcription/translation when incubated with protein kinase A catalytic subunit. 32P labeling of StAR protein expressed in COS-1 cells was also reduced when serine 57 or serine 195 were mutated to alanines. A double mutant in which both serine 57 and serine 195 were changed to alanines displayed markedly reduced 32P incorporation. To determine the functional significance of StAR phosphorylation, we tested the steroidogenic activity of the wild-type StAR and mutated StAR proteins in COS-1 cells expressing the human cholesterol side chain cleavage enzyme system. Mutation of the conserved protein kinase A phosphorylation site at serine 57 had no effect on pregnenolone synthesis. However, mutation of the serine residue at 195 resulted in an approximately 50% reduction in pregnenolone production. The S195A mutant construct did not yield the more acidic species of StAR detected in two-dimensional Western blots, indicating that the mutation affected the ability of the protein to be post-translationally modified. Mutation of the corresponding serine residues in murine StAR (Ser56 and Ser194) to alanines yielded results that were similar to those obtained with human StAR; the S56A mutant displayed a modest reduction in steroidogenic activity, whereas the S194A mutant had approximately 40% of the activity of murine wild-type StAR. In contrast to the human S195A mutation, conversion of serine 195 to an aspartic acid residue had no effect on steroidogenic activity, consistent with the idea that a negative charge at this site modulates StAR function. Our observations suggest that phosphorylation of serine 194/195 increases the biological activity of StAR and that this post- or co-translational event accounts, in part, for the immediate effects of cAMP on steroid production.

Localization of the steroidogenic acute regulatory protein in human tissues

The rate-limiting step in steroid hormone production in the adrenal cortex and gonads, the translocation of cholesterol from the outer to the inner mitochondrial membranes, is mediated by the steroidogenic acute regulatory protein (StAR). Heretofore, the localization of StAR in human adult and fetal tissues has not been defined. To this end, expression of StAR was detected in formalin-fixed, paraffin-embedded specimens using a polyclonal antiserum raised against recombinant human StAR. Primordial follicles of adult ovaries did not contain StAR, whereas antral follicles stained intensely in the thecal layer, with occasional staining of granulosa cells. Corpora lutea were intensely stained, but with a patchy distribution. Corpora albicantia did not stain. A luteoma of pregnancy stained with patches of moderate intensity. Ovaries with hyperthecosis contained areas of intense thecal staining. An ovarian Leydig cell tumor stained intensely, whereas granulosa cell tumors were negative. Ovarian adenocarcinomas, borderline tumors, teratomas, cystadenomas, and a Brenner tumor displayed no specific StAR immunostaining. Testicular Leydig cells stained moderately to intensely, as did a testicular Leydig cell tumor. Sertoli cells stained weakly in some specimens. Seminomas and testicular germ cell tumors were negative. There was minimal to moderate staining in the adrenal glomerulosa and faciculata and minimal staining in the reticularis, while the medulla was negative. Adrenal cortical adenomas, hyperplasias, and carcinomas all contained areas of StAR staining. The renal distal tubules stained with moderate to marked intensity. Renal carcinomas had occasional modest staining. No immunostaining was found in the placenta. Fetal ovaries contained sporadic stromal cells displaying intense StAR staining, particularly in the hilar region. Oocytes from a 32-week fetal ovary showed moderate to intense staining. Fetal testes displayed intense Leydig cell staining. The neocortex of the fetal adrenal glands displayed only minimal StAR staining, whereas moderate to intense staining was found in the fetal zone. The fetal kidneys had moderate StAR staining of the distal convoluted tubules. We conclude that StAR is localized to normal and neoplastic cells in the gonads and adrenal cortex, which produce large amounts of pregnenolone. StAR protein was not detected in the placenta, documenting that placental progestin synthesis occurs through StAR-independent mechanisms. The presence of StAR in cells that do not express cholesterol side-chain cleavage enzyme cytochrome P450, including renal distal tubules, Sertoli cells, and fetal oocytes, suggests that StAR has roles in metabolic processes in addition to stimulating pregnenolone synthesis.

MLN64 contains a domain with homology to the steroidogenic acute regulatory protein (StAR) that stimulates steroidogenesis

MLN64 is a protein that is highly expressed in certain breast carcinomas. The C terminus of MLN64 shares significant homology with the steroidogenic acute regulatory protein (StAR), which plays a key role in steroid hormone biosynthesis by enhancing the intramitochondrial translocation of cholesterol to the cholesterol side-chain cleavage enzyme. We tested the ability of MLN64 to stimulate steroidogenesis by using COS-1 cells cotransfected with plasmids expressing the human cholesterol side-chain cleavage enzyme system and wild-type and mutant MLN64 proteins. Wild-type MLN64 increased pregnenolone secretion in this system 2-fold. The steroidogenic activity of MLN64 was found to reside in the C terminus of the protein, because constructs from which the C-terminal StAR homology domain was deleted had no steroidogenic activity. In contrast, removal of N-terminal sequences increased MLN64's steroidogenesis-enhancing activity. MLN64 mRNA was found in many human tissues, including the placenta and brain, which synthesize steroid hormones but do not express StAR. Western blot analysis revealed the presence of lower molecular weight immunoreactive MLN64 species that contain the C-terminal sequences in human tissues. Homologs of both MLN64 and StAR were identified in Caenorhabditis elegans, indicating that the two proteins are ancient. Mutations that inactivate StAR were correlated with amino acid residues that are identical or similar among StAR and MLN64, indicating that conserved motifs are important for steroidogenic activity. We conclude that MLN64 stimulates steroidogenesis by virtue of its homology to StAR.

Analysis of the steroidogenic acute regulatory protein (StAR) gene in Japanese patients with congenital lipoid adrenal hyperplasia

Genomic DNA from 19 Japanese patients with congenital lipoid adrenal hyperplasia (lipoid CAH) representing 16 different families was examined to identify the genetic alterations of steroidogenic acute regulatory protein (StAR). Ten of 19 patients had a 46,XX karyotype and nine had a 46,XY karyotype. Six of the 46,XX patients have experienced spontaneous pubertal changes including breast development and irregular menstruation whereas none of the 46,XY subjects displayed pubertal changes. Eight different mutations were identified. Sixteen patients were either homozygotes or compound heterozygotes for the Q258X mutation. The seven other mutations identified were 189delG, 246insG, 564del13bp, 838delA, Q212X, A218V and M225T. The 189delG, 246insG, 546del13bp and Q212X mutants encode truncated proteins. COS-1 cells transfected with expression vectors encoding cDNAs for the mutant StAR proteins which affect the C-terminus, 838delA, A218V and Q258X, exhibited no steroidogenesis enhancing activity. However, the M225T mutant retained some steroidogenic activity. The patient with the M225T mutation had late onset of this disorder and some capacity to secrete testosterone in response to hCG. These findings suggest: (i) that the Q258X mutation can be used as a genetic marker for the screening of Japanese for lipoid CAH, (ii) that the C-terminus of StAR plays an important role in the protein's activity and (iii) that there are differences in the extent of functional impairment of the testis and ovaries in lipoid CAH.

Regulation of expression of the steroidogenic acute regulatory protein (StAR) gene: a central role for steroidogenic factor 1

Steroidogenic acute regulatory protein (StAR) plays a critical role in regulating the rate-limiting step in steroid hormone synthesis, cholesterol side-chain cleavage. StAR gene expression is transcriptionally controlled in the gonads by gonadotropic hormones via a cAMP second message. We have begun to analyze factors responsible for the transcriptional activation of the StAR gene. The human StAR gene promoter has at least two cis elements that govern basal and cAMP-regulated gene expression. One of these elements (the distal element) is a consensus binding sequence for the orphan nuclear receptor transcription factor, steroidogenic factor 1 (SF-1); the other (the proximal element) is a related motif. The human StAR promoter is not active in BeWo choriocarcinoma cells, but is functional and cAMP-responsive in murine Y1 adrenal cortical tumor cells. Cotransfection of a plasmid expressing SF-1 allows a StAR promoter construct to function in BeWo cells. Other orphan nuclear transcription factors do not support StAR promoter function in BeWo cell hosts. Deletion or mutation of the distal and proximal cis elements individually substantially reduces SF-1-supported StAR promoter activity. The distal site binds SF-1 with high affinity, whereas the proximal site binds SF-1 with lower affinities. These findings demonstrate a requirement for SF-1 for human StAR gene expression.

Steroidogenic acute regulatory protein (StAR) retains activity in the absence of its mitochondrial import sequence: implications for the mechanism of StAR action

Steroidogenic acute regulatory protein (StAR) plays a critical role in steroid hormone biosynthesis, presumably by facilitating the delivery of cholesterol to P450scc in the inner mitochondrial membranes. StAR is synthesized as a 37-kDa preprotein that is processed to a 30-kDa mature form by cleavage of an N-terminal mitochondrial import sequence. To identify structural features required for StAR biological activity, we mutated the human StAR cDNA, including the deletion of N- and C-terminal sequences, and examined the ability of the mutants to promote steroidogenesis and enter the mitochondria of transfected COS-1 cells. Deletion of up to 62 residues from the N terminus (N-62) did not significantly affect steroidogenesis-enhancing activity. The N-terminal deletion mutants were associated with mitochondria-enriched fractions, but import and processing were progressively impaired with increasing length of the deletion. Immunogold electron microscopy and in vitro import assays showed that the active N-62 mutant was not imported into the mitochondria. Removal of the 28 C-terminal amino acids (C-28) inactivated StAR. Deletion of the C-terminal 10 amino acids (C-10) reduced steroidogenic activity by 53%, while truncation of the last 4 amino acids had no effect. The C-28 mutant StAR was not efficiently imported into mitochondria or processed, whereas some of the C-10 mutant was processed, indicating that import had occurred. We conclude that in the COS-1 cell system used, StAR does not need to enter into mitochondria to stimulate steroidogenesis and that residues in the C terminus are essential for steroidogenesis-enhancing activity. These findings imply that StAR acts via C-terminal domains on the outside of the mitochondria.

Activated (HLA-DR+) T-lymphocyte subsets in early epithelial ovarian cancer and malignant ovarian germ cell tumors

We examined peripheral blood T-lymphocyte subsets before initiation of therapy in 79 healthy controls, 3 patients with endometriosis, 95 patients with common epithelial tumors of the ovary, 15 patients with ovarian germ cell tumors, and 3 patients with ovarian sex cord-stromal tumors. In stage Ia/Ib patients with epithelial ovarian cancer, the percentages of activated CD4+ (CD4+HLA-DR+) T cells and activated CD4+ T cells in the CD4+ T-cell subsets were significantly higher than those of healthy controls and patients with benign or borderline epithelial tumors of the ovary. These immunologic parameters were subsequently decreased in patients in stage Ic and more advanced stages. In malignant ovarian germ cell tumors, a similar increase in the CD4+ T-cell subsets was observed. Moreover, the percentage of activated CD8+ T cells in the CD8+ T-cell subsets in stage Ia/Ib patients increased significantly compared with those in healthy controls and patients with benign tumors. Our findings indicate that activated T lymphocytes may play some roles in oncogenesis and progression of both epithelial ovarian cancer and malignant ovarian germ cell tumors.

Phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) by proline-directed protein kinases and its dephosphorylation

We identified two major substrates for the proline-directed protein kinases--cdc2 kinase and tau protein kinase II (TPKII)--in the cytosol fraction from rat brains. The molecular masses of the proteins were 80 and 46 kDa. Because the 80-kDa protein was phosphorylated by protein kinase C and was heat stable, we examined the possibility that the protein might be myristoylated alanine-rich C kinase substrate (MARCKS). On the basis of a comparison between the properties of the 80-kDa protein and purified MARCKS, we concluded that the 80-kDa protein is indeed MARCKS. The amounts of phosphate incorporated into MARCKS by protein kinase C, cdc2 kinase, and TPKII were 1.7, 1.4, and 0.6 mol/mol of the protein, respectively. Two-dimensional tryptic peptide mapping indicated that phosphorylation sites by protein kinase C and proline-directed protein kinases completely differed. Only the seryl residue was phosphorylated by protein kinase C, whereas both seryl and threonyl residues were phosphorylated by cdc2 kinase and TPKII. Phosphorylation of MARCKS by protein kinase C inhibited the binding to calmodulin, whereas phosphorylation by cdc2 kinase and TPKII significantly increased the binding to calmodulin. The holoenzyme of protein phosphatase 2A dephosphorylated MARCKS that had been phosphorylated by protein kinase C, cdc2 kinase, or TPKII, whereas calcineurin was unable to dephosphorylate it. These results suggest that cdc2 kinase and TPKII regulate the functions of MARCKS in different ways from protein kinase C.

Stimulation of cyclic adenosine 3',5'-monophosphate-dependent protein kinase with brain gangliosides

The holoenzyme of cAMP-dependent protein kinase (cAMP-kinase) partially purified from the particulate fraction of rat brain was stimulated by gangliosides. Among various gangliosides tested, GM1 was most potent, giving Ka value of 19.5 microM. The maximal activation of the kinase was obtained with 100 microM GM1 using kemptide as substrate. Gangliosides inhibited the kinase activity of the catalytic subunit of cAMP-kinase. Of various substrates tested, the ganglioside-stimulated cAMP-kinase could phosphorylate microtubule-associated protein 2, synapsin I and myelin basic protein, but not histone H1 and casein. The molecular mechanisms of the stimulatory effect of gangliosides were investigated. The kinase activated with GM1 was inhibited by the addition of PKItide, a specific inhibitor for cAMP-kinase. However, GM1 did not dissociate the holoenzyme into the catalytic and regulatory subunits and did not interfere with the binding ability of cAMP to the holoenzyme. These results suggest that the gangliosides can directly activate cAMP-kinase in a different manner from cAMP.

Glycolipids isolated from Aplysia kurodai can activate cyclic adenosine 3',5'-monophosphate-dependent protein kinase from rat brain

Cyclic AMP (cAMP)-dependent protein kinase (cAMP-kinase) partially purified from the membrane fractions of rat brains was stimulated by novel phosphonoglycosphingolipids (glycolipids) derived from the skin and nerve fibers of Aplysia kurodai. Among various glycolipids tested, a major glycolipid from the skin, 3-O-MeGal beta 1-->3GalNAc alpha 1-->3[6'-O-(2-aminoethylphosphonyl)Gal alpha 1-->2](2-aminoethylphosphonyl-->6)Glc beta 1-->4Glc beta 1-->1ceramide (SGL-II), was most potent, giving half-maximal activation at 32.2 microM. Activation of cAMP-kinase was maximal with 250 microM SGL-II using kemptide as substrate. The effect of SGL-II was additive on kinase activity at submaximal concentrations of cAMP. The kinase activity activated with SGL-II was inhibited by the addition of protein kinase inhibitor peptide, a specific peptide inhibitor for cAMP-kinase. Its inhibitory pattern was similar to that for the catalytic subunit. Of the various substrates tested, the glycolipid-stimulated cAMP-kinase could phosphorylate microtubule-associated protein 2, synapsin I, and myelin basic protein but not histone H1 and casein. The regulatory subunit strongly inhibited the activity of purified catalytic subunit of cAMP-kinase. This inhibition was reversed by addition of SGL-II, as observed for cAMP. SGL-II was capable of partially dissociating cAMP-kinase, which was observed by gel filtration column chromatography. However, the binding activity of cAMP to the holoenzyme was not inhibited with SGL-II. These results demonstrate that the glycolipids can directly activate cAMP-kinase in a manner similar, but not identical, to that of cAMP.