Cyclin partners determine Pho85 protein kinase substrate specificity in vitro and in vivo: control of glycogen biosynthesis by Pcl8 and Pcl10

In Saccharomyces cerevisiae, PHO85 encodes a cyclin-dependent protein kinase (Cdk) with multiple roles in cell cycle and metabolic controls. In association with the cyclin Pho80, Pho85 controls acid phosphatase gene expression through phosphorylation of the transcription factor Pho4. Pho85 has also been implicated as a kinase that phosphorylates and negatively regulates glycogen synthase (Gsy2), and deletion of PHO85 causes glycogen overaccumulation. We report that the Pcl8/Pcl10 subgroup of cyclins directs Pho85 to phosphorylate glycogen synthase both in vivo and in vitro. Disruption of PCL8 and PCL10 caused hyperaccumulation of glycogen, activation of glycogen synthase, and a reduction in glycogen synthase kinase activity in vivo. However, unlike pho85 mutants, pcl8 pcl10 cells had normal morphologies, grew on glycerol, and showed proper regulation of acid phosphatase gene expression. In vitro, Pho80-Pho85 complexes effectively phosphorylated Pho4 but had much lower activity toward Gsy2. In contrast, Pcl10-Pho85 complexes phosphorylated Gsy2 at Ser-654 and Thr-667, two physiologically relevant sites, but only poorly phosphorylated Pho4. Thus, both the in vitro and in vivo substrate specificity of Pho85 is determined by the cyclin partner. Mutation of PHO85 suppressed the glycogen storage deficiency of snf1 or glc7-1 mutants in which glycogen synthase is locked in an inactive state. Deletion of PCL8 and PCL10 corrected the deficit in glycogen synthase activity in both the snf1 and glc7-1 mutants, but glycogen synthesis was restored only in the glc7-1 mutant strain. This genetic result suggests an additional role for Pho85 in the negative regulation of glycogen accumulation that is independent of Pcl8 and Pcl10.

Crystallographic studies of casein kinase I delta toward a structural understanding of auto-inhibition

A recombinant form of mammalian casein kinase I delta (CKIdelta) containing the catalytic domain and an auto-inhibitory domain was expressed in Escherichia coli, purified and crystallized. X-ray data were collected to 2.4 A resolution, and the crystals belong to space group C2221. Molecular replacement using the structure of the catalytic domain of CKIdelta yielded strong electron density for residues in the model, but no interpretable density was found for the inhibitory domain. A conserved intermolecular contact suggests the formation of dimers which would inhibit the activity of this protein kinase.

Comparison of thallium-201 and gallium-67 scintigraphy in the evaluation of non-Hodgkin's lymphoma

BACKGROUND

Gallium-67 (Ga-67) is widely used in the assessment of patients with lymphoma. Although highly sensitive in Hodgkin's disease and high grade non-Hodgkin's lymphoma (NHL), its sensitivity is reported to be reduced in lower grades of NHL. Several series have suggested that thallium-201 (Tl-201) may be more useful than Ga-67 in the evaluation of low and possibly intermediate grade NHL.

AIM

To compare the sensitivity and relative avidities (uptakes) of Tl-201 and Ga-67 in patients with NHL.

METHODS

A total of 36 pairs of studies were performed in 33 patients (14 male: 19 female; age range 16-84 years, mean 55 years). All had whole body Ga-67 (400 MBq) and Tl-201 (100-200 MBq) studies (interval 0-three days) which were read by two experienced nuclear medicine physicians blinded to clinical history and results of other investigations.

RESULTS

Tl-201 had a higher sensitivity and avidity than Ga-67 in patients with low grade NHL. In contrast, Ga-67 was slightly more sensitive and had a higher avidity in patients with both intermediate and high grade NHL.

CONCLUSIONS

We conclude that Tl-201 is more sensitive and avid for low grade NHL than Ga-67 and that consideration should be given to its use in this condition in conjunction with Ga-67 as it may provide complementary information. Routine use in intermediate grade NHL is less convincing as Ga-67 was superior in most cases. Ga-67 is more sensitive than Tl-201 in assessment of high grade NHL.

Glycogenin-2, a novel self-glucosylating protein involved in liver glycogen biosynthesis

Glycogenin is a self-glucosylating protein involved in the initiation phase of glycogen biosynthesis. A single mammalian gene had been reported to account for glycogen biogenesis in liver and muscle, the two major repositories of glycogen. We describe the characterization of novel forms of glycogenin, designated glycogenin-2 (GN-2), encoded by a second gene that is expressed preferentially in certain tissues, including liver, heart, and pancreas. Cloning of cDNAs encoding glycogenin-2 indicated the existence of multiple species, including three liver forms (GN-2alpha, GN-2beta, and GN-2gamma) generated in part by alternative splicing. Overall, GN-2 has 40-45% identity to muscle glycogenin but is 72% identical over a 200-residue segment thought to contain the catalytic domain. GN-2 expressed in Escherichia coli or COS cells is active in self-glucosylation assays, and self-glucosylated GN-2 can be elongated by skeletal muscle glycogen synthase. Antibodies raised against GN-2 produced in E. coli recognized proteins of Mr approximately 66,000 present in extracts of rat liver and in cultured H4IIEC3 hepatoma cells. In H4IIEC3 cells, most of the GN-2 was present as a free protein but some was covalently associated with glycogen fractions and was only released by treatment with alpha-amylase. H4IIEC3 cells also expressed the muscle form of glycogenin (glycogenin-1), which was attached to a chromatographically separable glycogen fraction.

Glucose-6-P control of glycogen synthase phosphorylation in yeast

The SNF1 gene encodes a protein kinase necessary for expression of glucose-repressible genes and for the synthesis of the storage polysaccharide glycogen. From a genetic screen, we have found that mutation of the PFK2 gene, which encodes the beta-subunit of 6-phosphofructo-1-kinase, restores glycogen accumulation in snf1 cells. Loss of PFK2 causes elevated levels of metabolites such as glucose-6-P, hyperaccumulation of glycogen, and activation of glycogen synthase, whereas glucose-6-P is reduced in snf1 cells. Other mutations that increase glucose-6-P, deletion of PFK1, which codes for the alpha-subunit of 6-phosphofructo-1-kinase, or of PGI1, the phosphoglucoisomerase gene, had similar effects on glycogen metabolism as did pfk2 mutants. We propose that elevated glucose-6-P mediates the effects of these mutations on glycogen storage. Glycogen synthase kinase activity was reduced in extracts from pfk2 cells but was restored to that of wild type if the extract was gel-filtered to remove small molecules. Also, added glucose-6-P inhibited the glycogen synthase kinase activity in extracts from wild-type cells, half-maximally at approximately 2 mM. We suggest that glucose-6-P controls glycogen synthase activity by two separate mechanisms. First, glucose-6-P is a direct activator of glycogen synthase, and second, it controls the phosphorylation state of glycogen synthase by inhibiting a glycogen synthase kinase.

Glycogen biogenesis in rat 1 fibroblasts expressing rabbit muscle glycogenin

Glycogenin, a self-glucosylating protein involved in the initiation of glycogen biosynthesis, varies in intracellular concentration from barely detectable in liver to a high level in muscle. The effect of increasing the glycogenin level on glycogen synthesis was studied in rat 1 fibroblasts stably overexpressing rabbit muscle glycogenin. In the presence of glucose, all of the expressed glycogenin was attached to polysaccharide and the free protein could only be detected by western blot analysis after incubation of cells in a glucose-depleted medium or treatment of the cell extract with alpha-amylase. In control cells, increased extracellular glucose concentrations promoted translocation of glycogen synthase from the soluble to the pellet fraction with an increase in the associated glycogen. Overexpression of glycogenin did not affect total intracellular glycogen and glycogen synthase levels at any concentration of glucose but significantly reduced glucose-induced accumulation of insoluble glycogen and translocation of glycogen synthase. Immunofluorescence analysis revealed a diffuse cytoplasmic distribution of glycogenin expressed in rat 1 cells. In rat 1 cells incubated with glucose, discrete deposits of glycogen were detected by staining with HIO4/Schiff but this was eliminated by overexpressing glycogenin. Analysis of [14C]glucose- or [35S]methionine-labeled extracts from glycogenin-expressing cells by continuous polyacrylamide gel electrophoresis and by two-dimensional gel electrophoresis revealed a continuum of glycogenin-containing species from low molecular mass to sizes significantly greater than 400 kDa. We conclude that (a) overexpression of glycogenin does not enhance glycogen synthesis but causes production of more, smaller, glycogen molecules with a concomitant change in their intracellular localization; (b) glycogenin and elevated glucose have opposing effects on the distribution of glycogenin and glycogen synthase in rat 1 cells; and (c) the biogenesis of glycogen in rat 1 cells occurs without the accumulation of any major intermediate form.

New insights into the role and mechanism of glycogen synthase activation by insulin

The metabolism of the storage polysaccharide glycogen is intimately linked with insulin action and blood glucose homeostasis. Insulin activates both glucose transport and glycogen synthase in skeletal muscle. The central issue of a long-standing debate is which of these two effects determines the rate of glycogen synthesis in response to insulin. Recent studies with transgenic animals indicate that, under appropriate conditions, each process can contribute to determining the extent of glycogen accumulation. Insulin causes stable activation of glycogen synthase by promoting dephosphorylation of multiple sites in the enzyme. A model linking this action to the mitogen-activated protein kinase signaling pathway via the phosphorylation of the regulatory subunit of glycogen synthase phosphatase gained widespread acceptance. However, the most recent evidence argues strongly against this mechanism. A newer model, in which insulin inactivates the enzyme glycogen synthase kinase-3 via the protein kinase B pathway, has emerged. Though promising, this model still does not completely explain the molecular basis for the insulin-mediated activation of glycogen synthase, which remains one of the many unknowns of insulin action.

Comparison of thallium-201 and gallium-67 scintigraphy in soft tissue and bone marrow multiple myeloma: a case report

A 68-year-old female was referred for assessment of multiple myeloma. A large myelomatous infiltrate involving the left triceps muscle showed avid uptake on both thallium-201 and gallium-67 scintigraphy. Following radiotherapy, imaging with both radiopharmaceuticals showed resolution of disease; however, tumour recurrence in the bone marrow was seen only on thallium-201 imaging. This observation suggests that while soft-tissue myeloma shows similar appearances on thallium-201 and gallium-67 scintigraphy, both at baseline and following therapy, gallium-67 may not demonstrate bone marrow infiltration which is visualized on thallium-201 imaging. Therefore, thallium-201 appears to be superior to gallium-67 in evaluation of patients with multiple myeloma when soft tissues and bone marrow are involved.

Technetium-99m-sestamibi scanning in recurrent medullary thyroid carcinoma

The presence of recurrent medullary thyroid carcinoma (MTC) can be detected early by measurement of serum calcitonin levels, but the localization of recurrent tumors is often difficult.

METHODS

We compared 99mTc-sestamibi scans with computed tomographic (CT) scans in 10 patients with recurrent MTC, who had basal serum calcitonin values ranging from 220-61800 ng/liter. Two patients additionally had bone scans performed because of the clinical suspicion of bone metastases.

RESULTS

Seven of the 10 patients had at least one site of abnormal 99mTc-sestamibi uptake, and all of these patients had basal serum calcitonin values > 6000 ng/liter. Only five of the 10 patients had abnormal CT scans. Technetium-99m-sestamibi scans detected 22 abnormal sites in the soft tissues of the neck and chest, while CT scans detected only 11 lesions in the neck and chest. Five of these sestamibi positive sites (in the neck and mediastinum of one patient) were confirmed histologically to represent MTC. When imaging the liver, CT scans detected 47 lesions in three patients while 99mTc-sestamibi scans detected none. One of these liver lesions was confirmed as MTC histologically. When imaging bone in two of the patients, the bone scans detected 17 abnormal sites, while 99mTc-sestamibi scans detected six abnormal sites.

CONCLUSION

Technetium-99m-sestamibi scans complement CT and bone scans in the localization of recurrent MTC in patients with extremely high calcitonin levels. Technetium-99m-sestamibi scans are more sensitive than CT scans in the assessment of the soft tissues of the neck and chest, but CT is more appropriate for imaging hepatic lesions and bone scans are better for imaging bone lesions. Technetium-99m-sestamibi scans are unlikely to be abnormal in patients with only mild elevation of calcitonin.

Comparison of optimised planar scintigraphy with SPECT thallium, exercise ECG and angiography in the detection of coronary artery disease

BACKGROUND

Thallium SPECT has been shown to be more sensitive than planar imaging in the detection of coronary heart disease (CAD) in a number of reported series. Early (< 10 minutes) redistribution on planar imaging has been demonstrated in clinical studies and this may partly contribute to its lower sensitivity.

AIM

To determine whether thallium SPECT is superior to planar scintigraphy (with the timing of imaging performed optimally so that it was commenced within five minutes of injection) in the detection of CAD.

METHODS

Planar and SPECT studies were performed in 44 patients with significant (> 70% stenosis) CAD, seven patients with borderline stenoses (50-69%) and in 18 patients with no significant CAD.

RESULTS

The sensitivity of planar imaging was 66% which was higher than exercise ECG 54% (ns) but significantly lower than SPECT 86% (p < 0.005). The specificity of planar thallium scintigraphy was 100% which was higher than SPECT (83%) and significantly higher than exercise ECG 72% (p < 0.05). SPECT had a significantly higher sensitivity for LAD and single vessel disease than planar imaging and this was unrelated to a history of prior myocardial infarction.

CONCLUSION

Even when planar imaging is timed optimally to minimise the impact of early redistribution, SPECT is more sensitive than either planar imaging or exercise ECG in the detection of CAD, but its specificity is lower.

Radionuclide cystography: a review of its use in adults

OBJECTIVE

Although a common procedure in pediatric nuclear medicine, direct radionuclide cystography is infrequently performed in adults in whom there are few reports on its use. We describe the use of direct radionuclide cystography in adults.

EXPERIMENTAL DESIGN

The medical records and direct radionuclide cystograms of 56 adults who underwent 75 studies at this institution over a 5 year period were reviewed.

SETTING

The study was performed at a major university teaching hospital.

PATIENTS

The major indications were for assessment of recurrent urinary tract infection and follow-up of known vesicoureteric reflux.

RESULTS

Overall, reflux was demonstrated in 36% patients. It was most frequently mild or moderate in severity and detected during both the filling and voiding phases of the study. Patients with neurogenic bladder dysfunction had a higher incidence of vesicoureteric reflux and significantly smaller functional bladder capacity than other referred patients. All studies were well tolerated, easily performed and without complication.

CONCLUSIONS

Given the lower radiation exposure and higher sensitivity than voiding cystourethrography, we believe that radionuclide cystography can be recommended in the evaluation of vesicoureteric reflux in adults as well as children. In the group of adults whose genitourinary anatomy is already known from other investigations, radionuclide cystography may be particularly suited.

Initiation of glycogen synthesis in yeast. Requirement of multiple tyrosine residues for function of the self-glucosylating Glg proteins in vivo

The self-glucosylating proteins, Glg1p and Glg2p, are required for glycogen synthesis in Saccharomyces cerevisiae (Cheng, C., Mu., J., Farkas, I., Huang, D., Goebl M. G., and Roach, P. J. (1995) Mol. Cell. Biol. 15, 6632-6640). Glg2p was shown to be associated with carbohydrate in vivo and was released from the high molecular weight glycogen fraction by treatment with alpha-amylase. In addition, some Glg2p exists as a protein of Mr approximately 43,000, whose proportion is increased in cells lacking glycogen synthase. Unlike the mammalian counterpart, glycogenin, the yeast Glg proteins appear to require multiple Tyr residues for functionality. In Glg2p, mutation of both Tyr230 and Tyr232 is necessary to suppress self-glucosylation of purified protein in vitro. The mutant protein is still capable of transferring glucose to an exogeneous acceptor, n-dodecyl beta-D-maltoside. A small COOH-terminal region, conserved between Glg1p and Glg2p, is also important for function; mutation of Tyr367 or truncation at residue 362 impairs the ability of primed Glg2p to be elongated by glycogen synthase. Complete suppression of glycogen accumulation in vivo requires mutation of all three Tyr residues. In Glg1p, two Tyr residues are implicated, Tyr232 and Tyr600, mutation of both being required to eliminate glycogen accumulation in vivo.

Bone lymphoma. Comparison of Tl-201 and Ga-67 citrate scintigraphy in assessment of treatment response

Assessment of response to therapy in patients with lymphoma involving bone can be difficult. Of the scintigraphic techniques available, Tl-201 may be the most ideal radiopharmaceutical because it reflects tumor burden more accurately than either Tc-99m MDP or Ga-67 citrate. We compared serial Tl-201 and Ga-67 citrate scintigraphs (24 studies, 12 pairs) in assessing the response to therapy in three patients with non-Hodgkin's lymphoma involving bone. In two patients who demonstrated a complete remission, uptake of Tl-201 diminished more rapidly and returned to normal earlier than did Ga-67 citrate. Tl-201 did not show tumor recurrence in the one patient with lesions shown to be Tl-201 negative at baseline. Serial studies using Tl-201 may be superior to Ga-67 citrate in the evaluation of lymphoma involving bone provided that Tl-201 avidity is demonstrated on baseline studies.

Pho85p, a cyclin-dependent protein kinase, and the Snf1p protein kinase act antagonistically to control glycogen accumulation in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, nutrient levels control multiple cellular processes. Cells lacking the SNF1 gene cannot express glucose-repressible genes and do not accumulate the storage polysaccharide glycogen. The impaired glycogen synthesis is due to maintenance of glycogen synthase in a hyperphosphorylated, inactive state. In a screen for second site suppressors of the glycogen storage defect of snf1 cells, we identified a mutant gene that restored glycogen accumulation and which was allelic with PHO85, which encodes a member of the cyclin-dependent kinase family. In cells with disrupted PHO85 genes, we observed hyperaccumulation of glycogen, activation of glycogen synthase, and impaired glycogen synthase kinase activity. In snf1 cells, glycogen synthase kinase activity was elevated. Partial purification of glycogen synthase kinase activity from yeast extracts resulted in the separation of two fractions by phenyl-Sepharose chromatography, both of which phosphorylated and inactivated glycogen synthase. The activity of one of these, GPK2, was inhibited by olomoucine, which potently inhibits cyclin-dependent protein kinases, and contained an approximately 36-kDa species that reacted with antibodies to Pho85p. Analysis of Ser-to-Ala mutations at the three potential Gsy2p phosphorylation sites in pho85 cells implicated Ser-654 and/or Thr-667 in PHO85 control of glycogen synthase. We propose that Pho85p is a physiological glycogen synthase kinase, possibly acting downstream of Snf1p.

Ictal and interictal technetium-99m-bicisate brain SPECT in children with refractory epilepsy

Identification of epileptogenic foci in patients with refractory epilepsy remains a significant diagnostic challenge. Magnetic resonance imaging studies frequently fail to reveal an anatomic origin for the seizures, and scalp electroencephalography is often limited to identification of the involved hemisphere. Functional imaging modalities such as PET and SPECT are more promising tools for this application because they reflect the functional pathology associated with the seizure. These changes are more pronounced ictally, but until recently, no radiopharmaceutical was available that could be used routinely for ictal SPECT. The present study was therefore undertaken to determine whether 99mTc-bicisate could be used in ictal SPECT in pediatric patients with refractory epilepsy, to compare the patterns of ictal and interictal blood flow in these patients and to compare the localization information provided by ictal SPECT with that available from other techniques.

METHODS

Technetium-99m-bicisate/SPECT was compared prospectively with scalp EEG for its ability to identify a possible seizure focus in pediatric patients with refractory epilepsy. Ictal and interictal SPECT studies were performed in 10 patients (3-19 yr old, mean age 10.9 +/- 4.3 yr; 7 female, 3 male) in whom MRI scans revealed no lesions that might be responsible for the seizures.

RESULTS

Ictal SPECT was performed in all patients, and all ictal studies revealed focal perfusion abnormalities. By comparison, four of the interictal SPECT studies showed regional hypoperfusion that corresponded to the regions of hyperperfusion in the ictal studies, and three showed regional hyperperfusion corresponding to the hyperperfused regions in the ictal studies. Three interictal studies revealed no abnormal perfusion. Scalp EEG provided localization information in five patients.

CONCLUSION

These initial results suggest that ictal SPECT with 99mTc-bicisate is a more promising tool for the identification of epileptogenic foci than interictal SPECT or scalp EEG in patients without focal abnormalities on MRI.

Genetic interactions between REG1/HEX2 and GLC7, the gene encoding the protein phosphatase type 1 catalytic subunit in Saccharomyces cerevisiae

Mutations in GLC7, the gene encoding the type 1 protein phosphatase catalytic subunit, cause a variety of abberrant phenotypes in yeast, such as impaired glycogen synthesis and relief of glucose repression of the expression of some genes. Loss of function of the REG1/HEX2 gene, necessary for glucose repression of several genes, was found to suppress the glycogen-deficient phenotype of the glc7-1 allele. Deletion of REG1 in a wild-type background led to overaccumulation of glycogen as well as slow growth and an enlarged cell size. However, loss of REG1 did not suppress other phenotypes associated with GLC7 mutations, such as inability to sporulate or, in cells bearing the glc7Y-170 allele, lack of growth at 14 degrees. The effect of REG1 deletion on glycogen accumulation is not simply due to derepression of glucose-repressed genes, although it does require the presence of SNF1, which encodes a protein kinase essential for expression of glucose-repressed genes and for glycogen accumulation. We propose that REG1 has a role in controlling glycogen accumulation.

Rate-determining steps in the biosynthesis of glycogen in COS cells

Consistent with previous results, overexpression of rabbit skeletal muscle glycogen synthase in COS cells did not lead to overaccumulation of glycogen unless activating Ser-->Ala mutations were present at key regulatory phosphorylation sites 2 (Ser7) and 3a (Ser644) in the enzyme. In addition, we found that expression of glycogenin, glycogen branching enzyme, or UDP-glucose pyrophosphorylase alone in COS cells had no effect on the glycogen level. However, coexpression of the hyperactive 2,3a glycogen synthase mutant with either glycogenin or UDP-glucose pyrophosphorylase led to higher glycogen accumulation than that obtained from the expression of glycogen synthase alone. Coexpression of glycogenin with the 2,3a mutant of glycogen synthase led to the appearance of glycogenin with a lower molecular weight suggestive of reduced glucosylation. Increased glycogen synthesis may lead to competition between glycogenin and glycogen synthase for their common substrate UDP-glucose. In summary, we conclude that (i) glycogen synthase is a primary rate-limiting enzyme of glycogen biosynthesis in COS cells, (ii) that phosphorylation of glycogen synthase is regulatory for glycogen accumulation, and (iii) once glycogen synthase is activated, the reaction mediated by UDP-glucose pyrophosphorylase can become rate-determining.

Three-dimensional structure of mammalian casein kinase I: molecular basis for phosphate recognition

The three-dimensional structure for the catalytic region of the mammalian protein kinase, casein kinase I delta (CKI delta), has been solved by X-ray crystallography to a resolution of 2.3 A. A truncation mutant of CKI delta lacking the C-terminal autoinhibitory region was expressed in Escherichia coli, purified, and crystallized. The structure was solved by molecular replacement using the crystal structure of the catalytic domain of a CKI homolog from Schizosaccharomyces pombe, Cki1. A tungstate derivative confirmed the initial molecular replacement solution and identified an anion binding site which may contribute to the unique substrate specificity of CKI. Like other protein kinases, the catalytic domain of CKI is composed of two lobes with a cleft between them for binding ATP. Comparison of the mammalian and yeast CKI structures suggests that a rotation of the N-terminal domain occurs upon ATP binding. This domain motion is similar, but not identical, to that observed in cAMP-dependent protein kinase upon binding ATP. Although Cki1 has many similarities to CKI delta over the catalytic domain, these two forms of CKI likely perform different functions in vivo. Relating the primary sequences of other CKI enzymes to the three-dimensional architecture of CKI delta reveals a catalytic face that is especially conserved among the subset of CKI family members associated with the regulation of DNA repair.

Bone metastases in pheochromocytoma. Optimal timing of I-123 MIBG scintigraphy

The optimal timing of I-123 MIBG scintigraphy in the evaluation of pheochromocytoma is unknown. Although some centers perform early imaging, many delay imaging until 24 hours after injection. The authors describe the case of a bony metastasis in the left femur that was detected 5 hours after injection, but which was not visualized at 24 hours.

Osteoid osteoma: comparative utility of high-resolution planar and pinhole magnification scintigraphy

A characteristic scintigraphic pattern of focally intense skeletal tracer uptake surrounded by less prominently increased uptake has been described for osteoid osteomas. Demonstration of this finding is useful in the evaluation of patients with suspected osteomas. Pinhole magnification scintigraphy was significantly more effective than high-resolution planar scintigraphy at revealing this pattern in 19 osteoid osteomas of the appendicular skeleton. A similar advantage was not observed for vertebral osteoid osteomas. The absence of the characteristic pattern aided in differentiating other pathology from osteoid osteomas in seven patients. The authors recommend that skeletal scintigraphy of patients with suspected osteoid osteomas include pinhole magnification imaging.

Multiple mechanisms for the phosphorylation of C-terminal regulatory sites in rabbit muscle glycogen synthase expressed in COS cells

Glycogen synthase can be inactivated by sequential phosphorylation at the C-terminal residues Ser652 (site 4), Ser648 (site 3c), Ser644 (site 3b) and Ser640 (site 3a) catalysed by glycogen synthase kinase-3. In vitro, glycogen synthase kinase-3 action requires that glycogen synthase has first been phosphorylated at Ser656 (site 5) by casein kinase II. Recently we demonstrated that inactivation is linked only to phosphorylation at site 3a and site 3b, and that, in COS cells, modification of these sites can occur by alternative mechanisms independent of any C-terminal phosphorylations [Skurat and Roach (1995) J. Biol. Chem. 270, 12491-12497]. To address these mechanisms multiple Ser-->Ala mutations were introduced in glycogen synthase such that only site 3a or site 3b remained intact. Additional mutation of Arg637-->Gln eliminated phosphorylation of site 3a, indicating that Arg637 may be important for recognition of site 3a by its corresponding protein kinase(s). Similarly, additional mutation of Pro645-->Ala eliminated phosphorylation of site 3b, indicating a possible involvement of 'proline-directed' protein kinase(s). Mutation of Arg637 alone did not activate glycogen synthase as expected from the loss of phosphorylation at site 3a. Rather, mutation of both Arg637 and the Ser-->Ala substitution at site 3b was required for substantial activation. The results suggest that sites 3a and 3b can be phosphorylated independently of one another by distinct protein kinases. However, phosphorylation of site 3b can potentiate phosphorylation of site 3a, by an enzyme such as glycogen synthase kinase-3.