Intracellular localization of inositol-phospholipid-metabolizing enzymes in rabbit fast-twitch skeletal muscle. Can D-myo-inositol 1,4,5-trisphosphate play a role in excitation-contraction coupling?

Human phosphatidylinositol 4-kinase isoform PI4K92. Expression of the recombinant enzyme and determination of multiple phosphorylation sites

Human phosphatidylinositol 4-kinase, isoform PI4K92, was expressed as His6 tagged protein in Sf9 cells reaching a level of approximately 5% of cellular protein. The enzyme can be purified nearly to homogeneity in a single step by absorption/desorption on Ni/nitriloacetic acid agarose magnetic beads. High Km values in the millimolar range for ATP and PtdIns as well as only a moderate inhibition by adenosine and a sensitivity to Wortmannin (IC50 approximately 300 nM) characterize the enzyme as a type 3 PI4K. The enzyme produces PtdIns4P as product. The isolated enzyme is a phosphoprotein, additionally phosphate is incorporated by incubation with ATP/Mg or ATP/Mn. Phosphorylation sites were mapped by MALDI-MS and LC-MS/MS at the following positions: S258, T263, S266, S277, S294, T423, S496, T504. Accordingly, a stretch of 81 amino acids between the common and the C-terminal catalytic domain was designated phosphorylation domain.

The ATP-binding site of brain phosphatidylinositol 4-kinase PI4K230 as revealed by 5'-p-fluorosulfonylbenzoyladenosine

The ATP-binding site of purified bovine brain phosphatidylinositol 4-kinase 230 (PI4K230) was studied by its reaction with 5'-p-fluorosulfonylbenzoyladenosine (FSBA), an ATP-like alkylating reagent. Four hundred to eight hundred micromolar FSBA inactivated PI4K230 specifically with apparently first-order kinetics and resulted in 50% loss of enzyme activity in 36--130 min. The specificity of the reaction with FSBA was demonstrated by the lack of inactivation with 5'-p-fluorosulfonylbenzoyl chloride and by protection with ATP and ATP analogues against inactivation. Most ATP analogues competed with FSBA inactivation in order of their increasing hydrophobicity, parallel to their inhibitory potency in activity measurements. The specific binding of FSBA to PI4K230 was demonstrated also by Western-blot experiments. These results suggest that FSBA-reactive group(s) involved in the enzyme activity are located near to the ATP-binding site in a hydrophobic region of native PI4K230. Experiments with site-directed mutagenesis indicate that the conserved Lys-1792 plays essential role in the enzyme activity and serves as one target of affinity labelling by FSBA. Prevention of both Lys-1792-directed and Lys-1792-independent binding of FSBA by Cibacron Blue 3GA suggest that these sites are located spatially close to each other.

Synthesis of new cyclitol compounds that influence the activity of phosphatidylinositol 4-kinase isoform, PI4K230

The synthesis, chemical derivatization, and investigation of the inhibitory properties of novel cyclitol derivatives on the phosphatidylinositol 4-kinase enzymes PI4K55 and PI4K230 involved in the phosphatidylinositol cycle are reported. Some of the prepared cyclitol derivatives (i.e. 9, 11, 12, and 14) proved to be very powerful and specific irreversible inhibitors of PI4K230 at or below a concentration of 1 mM.

Functional expression and characterisation of a new human phosphatidylinositol 4-kinase PI4K230

By constructing DNA probes we have identified and cloned a human PtdIns 4-kinase, PI4K230, corresponding to a mRNA of 7.0 kb. The cDNA encodes a protein of 2044 amino acids. The C-terminal part of ca. 260 amino acids represents the catalytic domain which is highly conserved in all recently cloned PtdIns 4-kinases. N-terminal motifs indicate multiple heterologous protein interactions. Human PtdIns 4-kinase PI4K230 expressed in vitro exhibits a specific activity of 58 micromol mg-1min-1. The enzyme expressed in Sf9 cells is essentially not inhibited by adenosine, it shows a high Km for ATP of about 300 microM and it is half-maximally inactivated by approximately 200 nM wortmannin. These data classify this enzyme as type 3 PtdIns 4-kinase. Antibodies raised against the N-terminal part moderately activate and those raised against the C-terminal catalytic domain inhibit the enzymatic activity. The coexistence of two different type 3 PtdIns 4-kinases, PI4K92 and PI4K230, in several human tissues, including brain, suggests that these enzymes are involved in distinct basic cellular functions.

Changes in IP3 metabolism during skeletal muscle development in vivo and in vitro

We have investigated whether IP3 metabolism presents particular changes during critical stages of muscle development. With this aim, we have measured IP3 formation through phospholipase C activity, IP3 removal through IP3 5-phosphatase and IP3 3-kinase activities, as well as IP3 mass, during myogenesis in vivo and in vitro. In developing rat skeletal muscle, both IP3 3-kinase and 5-phosphatase activities were relatively constant from embryonary day 15, the earliest age studied to postnatal day 10; 5-phosphatase decreased upon further development. A transient, major increase in phospholipase C activity was evident at embryonary day 18 while a non-significant increase in IP3 mass was detected at this embrionary age. In rat skeletal muscle in primary culture, all enzyme activities as well as the mass of IP3 increased significantly in myotubes compared to myoblasts. Myotubes incubated with calcitonin gene-related peptide, responded with a transient increase in IP3 mass after 2 to 10 sec; the CGRP-induced increase being completely blocked by U-73122, a phospholipase C inhibitor. Furthermore, IP3 mass increased within 1 hr after exposure to differentiating agents of both RCMH cells, a line derived from normal human skeletal muscle, and C2C12 cells. These results indicate that changes in IP3 metabolism can be correlated to critical stages of muscle development and differentiation, suggesting a possible role for IP3 in these processes.

Identification of a 200 kDa polypeptide as type 3 phosphatidylinositol 4-kinase from bovine brain by partial protein and cDNA sequencing

Two phosphatidylinositol 4-kinase isozymes, type 3 and type 2, have been separated on hydroxylapatite after solubilizing bovine brain microsomes with Triton X-114. Employing a newly developed renaturation procedure following SDS-PAGE, we demonstrate that a 200 kDa polypeptide carries the enzyme activity of this type 3 isoform. Chromatography on hydroxylapatite, Heparin-Sepharose, Superdex 200 and finally SDS-PAGE results in an approximately 30,000-fold purification. Tryptic peptides generated from the 200 kDa polypeptide after SDS-PAGE have been sequenced and the obtained data have been used for constructing and synthesizing degenerated oligonucleotides. Polymerase chain reaction as well as screening of cDNA libraries allowed several clones to be isolated from which a 4.7 kb contiguous sequence can be built up. The open reading frame covers 4.4 kb with a 0.3 kb untranslated 3' end which yields a deduced amino acid sequence of 1,467 amino acids. The C-terminal part of ca. 300 amino acids represents the catalytic domain. Sequence alignment of this domain with the mammalian counterpart, the human type 2 phosphatidylinositol 4-kinase, the yeast kinases STT4 and PIK1, as well as with the catalytic domains of bovine, human, mouse and yeast phosphatidylinositol 3-kinases reveals a high degree of identity: 26 of these approximately 300 amino acids are invariable in all of these eight catalytic domains. Five motifs indicate nuclear localization and DNA binding properties of the enzyme. Two leucine zipper motifs (amino acids 358-386, 862-882) are detectable. Furthermore, a helix loop helix motif (amino acids 716-729) as well as two nuclear localization signals (amino acids 838-854, 345-349) indicate the presence of the type 3 isoform in the nucleus.

Cloning of a phospholipase C-delta 1 of rabbit skeletal muscle

The phospholipase C isoform responsible for the increase in the total myoplasmic inositol 1,4,5-trisphosphate concentration during tetanic contraction of isolated skeletal muscle and its mechanism of activation is not known. We have cloned and sequenced a phospholipase C cDNA of rabbit skeletal muscle coding for a protein of 745 amino acids with a molecular mass of 84,440 kDa. The deduced amino acid sequence exhibits the phospholipase C-specific domains X and Y which according to current knowledge very likely represent the catalytic centre of the enzyme. An overall sequence homology of 88% to the phospholipase C-delta 1 of rat brain suggests that the encoded protein represents a phospholipase C-delta 1 isoform of rabbit skeletal muscle. Northern blot analysis shows, that this phospholipase C-delta is dominantly expressed in skeletal muscle, less strongly in smooth muscle (uterus) and lung and weakly in heart, kidney and brain. In the N-terminal part of the primary structure a consensus sequence for a canonical EF-hand Ca2+ binding domain can be identified together with a short positively charged motif which recently has been suggested to be essential for the binding of phosphatidylinositol 4,5-bisphosphate. If these two domains which are unique for phospholipase C-delta are sufficient in establishing a mechanism for the activation of the enzyme, inositol 1,4,5-trisphosphate formation in skeletal muscle could be the consequence of an increase in myoplasmic Ca2+.

Effects of fatigue of rat EDL in situ on metabolism of phosphoinositides

The study was conducted to determine the effect of persistent fatigue in situ on the inositol phosphate second messenger system proposed to constitute a step in excitation-contraction coupling. Rat EDL, after stimulation in situ for 1 hr (100 Hz for 330 ms, 1/s), showed increased incorporation of myo-[3H]inositol into membrane phosphoinositides during a subsequent 4-h incubation period. The rate of hydrolysis of this pool resulting from 10 sec of tetanic stimulation, as well as the rate of production of inositol phosphates InsP, InsP2, and InsP3, were significantly reduced in fatigued muscles. These results suggest that the metabolic changes that parallel the alteration in contractile response with fatigue reflect a disruption in E-C coupling.

Phosphatidylinositol 4-phosphate increases the rate of dephosphorylation of the phosphorylated Ca(2+)-ATPase

Incubation of the Ca(2+)-ATPase of skeletal muscle sarcoplasmic reticulum with ATP in the absence of Ca2+ leads to phosphorylation of phosphatidylinositol (PtdIns) to phosphatidylinositol 4-phosphate (PtdIns-4P) and to a doubling of ATPase activity. Similarly, reconstitution of the ATPase with mixtures of dioleoylphosphatidylcholine and PtdIns-4P also led to a doubling of activity; ATPase activity increased with increasing PtdIns-4P content, up to 10% beyond which no further increase was observed. Reconstitution with PtdIns had a much smaller effect on activity. Changes in the Ca2+ affinity of the ATPase following incubation with ATP or reconstitution with PtdIns-4P were small. The rates of phosphorylation of the ATPase by ATP and of the Ca2+ transport step were unaffected, but the rate of dephosphorylation of the phosphorylated ATPase increased by a factor of 2 either following incubation with ATP or following reconstitution with PtdIns-4P. Activation of the ATPase led to a decrease in the level of phosphorylation of the ATPase by Pi corresponding to a 10-fold decrease in the equilibrium constant E2PMg/E2PiMg.

Inositol 1,4,5-trisphosphate 3-kinase activity in frog skeletal muscle

Frog skeletal muscle contains a kinase activity that phosphorylates inositol 1,4,5-trisphosphate to inositol 1,3,4,5-tetrakisphosphate. The inositol 1,4,5-trisphosphate 3-kinase activity was mainly recovered in the soluble fraction, where it presented a marked dependency on free calcium concentration in the physiological range in the presence of endogenous calmodulin. At pCa 5, where the activity was highest, the soluble 3-kinase activity displayed a Km for inositol 1,4,5-trisphosphate of 1.6 microM and a Vmax value of 25.1 pmol mg-1 min-1. The removal rates of inositol 1,4,5-trisphosphate by 3-kinase and 5-phosphatase activities of the total homogenate under physiological ionic conditions were very similar, suggesting that both routes are equally important in metabolizing inositol 1,4,5-trisphosphate in frog skeletal muscle.

The metabolism of D-myo-inositol 1,4,5-trisphosphate and D-myo-inositol 1,3,4,5-tetrakisphosphate by porcine skeletal muscle

In soluble and particulate extracts from muscle D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and D-myo-inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] are metabolised stepwise to inositol. Ins(1,4,5)P3 is rapidly dephosphorylated to D-myo-inositol 1,4-bisphosphate then to D-myo-inositol 4-phosphate and finally inositol. In soluble extracts Ins(1,3,4,5)P4 is dephosphorylated to D-myo-inositol 1,3,4-trisphosphate then sequentially to D-myo-inositol 3,4-bisphosphate, D-myo-inositol 3-phosphate and inositol, while in particulate extracts D-myo-inositol 1,3-bisphosphate is the predominant inositol bisphosphate formed. Dephosphorylation of these inositol polyphosphates is Mg2+ dependent and inhibited by D-2,3-bisphosphoglyceric acid. Ins(1,4,5)P3 is also phosphorylated to form Ins(1,3,4,5)P4 in soluble extracts by Ins(1,4,5)P3 3-kinase. Ins(1,4,5)P3 3-kinase activity is Mg2+ and ATP dependent and is stimulated by Ca2+ and calmodulin. Particulate (sarcotubular) inositol polyphosphate 5-phosphatase (5-phosphatase) is found in membranes which are intimately involved in excitation-contraction coupling and the generation of the primary Ca2+ signal of muscle cells. Particulate 5-phosphatase had the highest specific activity in the transverse-tubule membrane, when compared to the terminal cisternae and longitudinal-tubule membranes of the sarcoplasmic reticulum. Particulate Ins(1,3,4,5)P4-3-phosphatase activity was also detected after fractionation of solubilised sarcotubular membranes by DEAE-Sephacel. Particulate 5-phosphatase activity was purified 25,600-fold to a specific activity of 25.6 mumol Ins(1,4,5)P3 hydrolysed.min-1.mg protein-1, after DEAE-Sephacel and novel affinity chromatography using D-2,3-bisphosphoglycerate/agarose and Sepharose-4B-immobilised Ins(1,4,5)P3-analog matrices. Purified particulate 5-phosphatase had apparent Km of 46.3 microM and 1.9 microM and Vmax of 115 and 0.046 mumol substrate hydrolysed.min-1.mg protein-1, for Ins(1,4,5)P3 and Ins(1,3,4,5)P4, respectively. In contrast, purified soluble type I 5-phosphatase had apparent Km of 8.9 microM and 1.1 microM and Vmax of 3.55 and 0.13 mumol substrate hydrolysed.min-1.mg protein-1, for Ins(1,4,5P3 and Ins(1,3,4,5)P4, respectively. As in other cells, muscle 5-phosphatases have a lower affinity, but a higher capacity to metabolise Ins(1,4,5)P3 than Ins(1,3,4,5)P4. Soluble type I 5-phosphatase may have a functional role in the metabolism of both inositol polyphosphates, while particulate 5-phosphatase may primarily metabolise Ins(1,4,5)P3. Purified Ins(1,4,5)P3 3-kinase had an apparent Km of 0.42 microM and a Vmax of 4.12 nmol Ins(1,4,5)P3 phosphorylated.min-1.mg protein-1. The profile of inositol polyphosphate metabolism in muscle is similar to that reported in other tissues.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphoinositides in membranes that build up the triads of rabbit skeletal muscle

The total membrane concentrations of PtdIns, PtdIns4P, and PtdIns(4,5)P2 contribute to the functional capacity of the Ins(1,4,5)P3 signalling system which is operating in skeletal muscle but the function of which is still unknown. Total amounts of these phosphoinositides have been determined in purified membranes of transverse tubules (TT) and terminal cisternae (TC) of the sarcoplasmic reticulum (SR) of rabbit skeletal muscle. PtdIns and PtdIns4P have been detected in both membrane systems whereas PtdIns(4,5)P2 (290 mumol/mol phospholipid) is confined only to TT. A much greater pool of PtdIns(4,5)P2 seems, however, to be located in the sarcolemma away from the triadic junction.

The role of phosphoinositide metabolism in Ca2+ signalling of skeletal muscle cells

1. The mobilization of Ca2+ from intracellular stores by D-myo-inositol 1,4,5-triphosphate[Ins(1,4,5)P3] is now widely accepted as the primary link between plasma membrane receptors that stimulate phospholipase C and the subsequent increase in intracellular free Ca2+ that occurs when such receptors are activated (Berridge, 1993). Since the observations of Volpe et al. (1985) which showed that Ins(1,4,5)P3 could induce Ca2+ release from isolated terminal cisternae membranes and elicit contracture of chemically skinned muscle fibres, research has focused on the role of Ins(1,4,5)P3 in the generation of SR Ca2+ transients and in the mechanism of excitation-contraction coupling (EC-coupling). 2. The mechanism of signal transduction at the triadic junction during EC-coupling is unknown. Asymmetric charge movement and mechanical coupling between highly specialized triadic proteins has been proposed as the primary mechanism for voltage-activated generation of SR Ca2+ signals and subsequent contraction. Ins(1,4,5)P3 has also been proposed as the major signal transduction molecule for the generation of the primary Ca2+ transient produced during EC-coupling. 3. Investigations on the generation of Ca2+ transients by Ins(1,4,5)P3 have been conducted on ion channels incorporated into lipid bilayers, skinned and intact fibres and isolated membrane vesicles. Ins(1,4,5)P3 induces SR Ca2+ release and the enzymes responsible for its synthesis and degradation are present in muscle tissue. However, the sensitivity of the Ca2+ release mechanism to Ins(1,4,5)P3 is highly dependent on experimental conditions and on membrane potential. 4. While Ins(1,4,5)P3 may not be the major signal transduction molecule for the generation of the primary Ca2+ signal produced during voltage-activated contraction, this inositol polyphosphate may play a functional role as a modulator of EC-coupling and/or of the processes of myoplasmic Ca2+ regulation occurring on a time scale of seconds, during the events of contraction.

Phospholipase C activity in membranes and a soluble fraction isolated from frog skeletal muscle

Highly purified triads and transverse tubules, as well as soluble fraction isolated from frog skeletal muscle, hydrolyze exogenous phosphatidylinositol 4,5-bisphosphate forming inositol 1,4,5-trisphosphate with maximal rates in the range 0.5-1 nmol/mg per min at pCa 3. Sarcoplasmic reticulum membranes present a minor activity. The hydrolysis rates in triads were 0.072 +/- 0.015 nmol/mg per min at pCa 7, increasing to 0.263 +/- 0.026 nmol/mg per min at pCa 5 with 1.0 mM Mg and 0.1 mM substrate. The phospholipase C activity of isolated transverse tubules at pCa 3 was 0.570 +/- 0.032 nmol/mg per min. Since triads contain 10% transverse tubules, and correcting for the small contribution of sarcoplasmic reticulum, the calculated phospholipase C activity of transverse tubules at pCa 3 is about 10-times higher than the observed values, suggesting loss of activity during isolation. The activation by calcium was also observed in a soluble fraction and was neither replaced nor inhibited by magnesium. No effect of GTP analogs on phospholipase C activity was detected.

Phosphoinositides in giant barnacle muscle fibers: a quantitative analysis at rest and following electrical stimulation

Quantitative data are presented on the composition of the major phospholipids in isolated giant barnacle muscle fibers. It is shown, using internal perfusion techniques, that the high specific activity of labeling of polyphosphoinositides in vivo is attained by the activities of specific kinases. Electrical stimulation causes a reduction in the specific activity of labeling of PtdInsP2. This phospholipid, which is the immediate precursor for the release of InsP3, is found at a significant concentration (40 nmol/g wet weight) in single barnacle muscle fibers, sufficient to support a role as precursors of second messengers. The rapid catabolization of PtdInsP2 in the absence of external Ca2+ suggests that E-C coupling in barnacle muscle may be associated with a voltage-dependent, Ca(2+)-independent, activation of the breakdown of polyphosphoinositides.

Rabbit fast skeletal muscle phospholipase C. Molecular weight determination by renaturation after polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate

Phosphoinositide specific phospholipase C from rabbit fast skeletal muscle has been enriched ca. 1,000-fold with a specific activity of 40 mumol x min-1 x mg-1. Following SDS-PAGE, renaturation of the enzyme protein in the presence of deoxycholate allowed the determination of an apparent molecular weight of 110 kDa. Gel-filtration of the native enzyme resulted in a very similar apparent molecular weight of 115 kDa, however, associated proteins of higher molecular weight were also found. Free Ca2+ concentrations needed for half-maximal activation of PtdIns(4,5)P2, PtdIns4P and PtdIns hydrolysis are 6.3 microM, 85 microM and 1.8 mM, and the Km values for these substrates 102, 340 and 937 microM, respectively.

The significance of Na+ in E-C coupling in muscle

Recent studies have revealed arguments in favour of the possible triggering role of Na+ ions in E-C coupling in skeletal muscle fibres of vertebrates: (i) Na+ is one of the four major inorganic cations widespreaded in the biosphere ununiformly (with gradient) distributed across plasmic membranes of all muscle fibres; (ii) there is correlation between contractile parameters and a pattern of transsarcolemmal Na+ distribution in skeletal muscles; (iii) "Na+ current-induced Ca2+ release" mechanism is corresponded to the criterions for intracellular mediators: a) excitation of plasmic membrane increases [Na+] in junctional space; b) increase of [Na+] in surroundings in vitro induces efflux of Ca2+ from SR; c) estimated rate of [Na+] increase in junctional space in vivo is exceeded the threshold that induces Ca2+ from the SR in vitro; d) there is endogenic system (Na+, K(+)-ATPase) of quick removal of Na+ from junctional gap of triads; e) pharmacological modification of Na+ current through T-tubule membrane leads to correlated changing in twitch response. A definite order of Na(+)- and Ca2+ transmembrane triggering fluxes involved in E-C coupling in fast skeletal muscle fibers provide a very protective intracellular functional system of Ca2+ regulation, coordinated in time and space, and garantees the most complete dependence of voluntary muscle contractions on the CNS control.

Masses of inositol phosphates in resting and tetanically stimulated vertebrate skeletal muscles

The masses of inositol phosphates have been determined in isolated skeletal muscles from Xenopus laevis (sartorius, tibialis anterior and iliofibularis) and rat (gastrocnemius and soleus) which were quick-frozen in the resting state and at different stages of an isometric (Xenopus) or isotonic (rat) tetanus. The isomeric spectrum of inositol phosphates detected was similar to that in other tissues and cell types. The total sarcoplasmic concentrations of the isomers Ins-(1,4,5,6)P4/Ins(3,4,5,6)P4 (0.2-0.9 microM), Ins(1,3,4,6)P4 (not detectable), Ins(1,3,4,5,6)P5 (about 1 microM) and InsP6 (3.2-4.6 microM) were lower than in other cell types. Variations in these concentrations were due to the muscle type rather than to the donor species. The putative second messenger Ins(1,4,5)P3, as well as its dephosphorylation product Ins(1,4)P2, were present at surprisingly high total myoplasmic resting concentrations, ranging from 1.2 to 2.5 microM and 3.5 to 6.9 microM respectively. Upon tetanic stimulation these two inositol phosphates in particular exhibited significantly increased total sarcoplasmic concentrations, up to 4.2 microM and 11.3 microM respectively, with a time scale of seconds. From the initial rate of increase in the total sarcoplasmic concentrations of Ins(1,4,5)P3 and its rapidly formed metabolic products, a minimal phosphoinositidase C (PIC) activity in tetanically activated Xenopus skeletal muscle of about 1.7-2.6 microM/s can be estimated. This PIC activity observed in vivo seems to be far too low to account for a functional role for Ins(1,4,5)P3 as a chemical transmitter in the fast excitation-contraction coupling (ECC) process in skeletal muscle. The presence of Ins(1,3,4,5)P4 in all muscle types is indicative of a Ca(2+)-activated Ins(1,4,5)P3 3-kinase activity. The rapid transient increases in Ins(1,3,4)P3 and Ins(1,3)P2 in isometrically contracting Xenopus muscles suggest that corresponding Ins(1,3,4,5)P4 phosphatases are operating in skeletal muscle as well. In all muscles investigated except rat soleus, the fructose 1,6-bisphosphate [Fru(1,6)P2] concentration increased substantially during a tetanus, up to about 2 mM. This increase is correlated with a simultaneous decrease in phosphocreatine, whereas the energy charge of the muscles was essentially unaffected by the applied tetani. The time course of the rise in Fru(1,6)P2 was used to model changes in the free concentrations of high-affinity aldolase-binding inositol phosphates during the course of a tetanus. These calculations demonstrate that the free concentration of Ins(1,4,5)P3 and other aldolase-bound inositol phosphates can increase much faster and to a larger extent than the corresponding total concentrations as a result of their competitive displacement from aldolase-binding sites by the rapidly rising concentration of Fru(1,6)P2.

Inositol 1,4,5-triphosphate phosphatase activity in membranes isolated from amphibian skeletal muscle [corrected]

The hydrolysis of [3H]inositol 1,4,5-trisphosphate by a soluble fraction and by isolated transverse tubule and sarcoplasmic reticulum membranes from frog skeletal muscle was studied. Transverse tubule membranes displayed rates of hydrolysis several-fold higher than those of sacroplasmic reticulum and soluble fraction; Km and Vmax were 25.2 microM and 44.1 nmol/mg/min, respectively. Transverse tubule membranes sequentially hydrolyzed inositol trisphosphate to inositol bisphosphate, inositol 1-phosphate and inositol, indicating that these membranes have inositol bis- and monophosphatases in addition to inositol trisphosphatase.

Synthesis of polyphosphoinositides in transverse tubule and sarcoplasmic reticulum membranes of frog skeletal muscle

Synthesis of polyphosphoinositides has been studied in transverse (T-) tubule and sarcoplasmic reticulum (SR) membrane fractions of frog skeletal muscle, following 32P-labeling with [gamma-32P]ATP. Purified SR and T-tubule fractions respectively synthesize 9.4 +/- 0.8 and 71.9 +/- 9.8 pmol PtdInsP/mg per min, indicating nearly 8-fold higher activity of PtdIns kinase in the T-tubules than in the SR. The activity of this enzyme in both membrane systems is maximum at pH 7 and pCa 6. PtdInsP2 is synthesized from the endogenous PtdInsP, only in T-tubule membranes by the action of PtdInsP kinase. This lipid is the most intensely 32P-labeled phosphoinositide (181.7 +/- 9.2 pmol/mg per min) in these membranes. PtdIns kinase in the T-tubule and SR membranes, and PtdInsP kinase in the former are modulated by the free [Mg2+]. Loss of radiolabel from transiently maximal 32P-incorporation in polyphosphoinositides in T-tubule membranes, concomitant with a decrease in the ATP concentration in the incubation buffer, shows the occurrence of phosphoinositidases in these membranes. Under the conditions used, no such activities were evident in SR membranes. Compound 48/80, a mixture of condensation products of N-methyl-p-methoxyphenethylamine with formaldehyde, known to block phosphoinositidase C and phospholipase A2, causes a dose-dependent increase in the 32P-label of PtdInsP, in T-tubule membranes. The synthesis of lyso PtdInsP2, a deacylated form of PtdInsP2 which occurs in nearly equal quantities in both T-tubule and SR membranes, may result from a mechanism independent of phospholipase A2.

Relation of phosphatidylinositol metabolism to glycolytic pathway in skeletal muscle membranes

Skeletal muscle triads are possessing the whole set of enzymes of the phosphatidylinositol (PI)-linked signal generating pathway, PI-kinase, PI(4)P-kinase, and PI(4,5)P2-phospholipase C (PLC). The activities of these enzymes are comparable to those found in other cell types for which a functional role of the PI-pathway in intracellular signal transduction has been established. For skeletal muscle an unequivocal function and an initiating signal for Ins(1,4,5)P3-liberation is still unknown. However, the observed Ca-dependency of PLC activity suggests that here Ins(1,4,5)P3 production is a consequence rather than a cause of increasing cytosolic Ca2+. Recently, the glycolytic enzyme aldolase, whose activity can be modulated by inositol polyphosphates, has been localized in the triadic structure. The enzyme which has a high affinity to Ins(1,4)P2, Ins(1,4,5)P3 and Ins(1,3,4,5)P4, seems to be compartmentalized to the junctional foot structure from which it is released upon binding of these molecules. This phenomenon could reflect a capability for regulation of the glycolytic flux even for aldolase, especially if a non steady-state situation in the junctional gap is considered. Meanwhile we have accumulated evidence for the operation of a partial glycolytic sequence in the junctional region established by the enzymes aldolase, glyceraldehyde-3-P (GAP) dehydrogenase and phosphoglycerate kinase. This system is able to produce ATP upon oxidation of GAP and could be, because of the inositol polyphosphate-sensing abilities of aldolase, a target for the membrane associated PI-pathway. The ATP production is however transient which indicates the coupling to an ATP hydrolyzing reaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and functional aspects of calcium homeostasis in eukaryotic cells

The maintenance of a low cytosolic free-Ca2+ concentration, ([Ca2+]i) is a common feature of all eukaryotic cells. For this purpose a variety of mechanisms have developed during evolution to ensure the buffering of Ca2+ in the cytoplasm, its extrusion from the cell and/or its accumulation within organelles. Opening of plasma membrane channels or release of Ca2+ from intracellular pools leads to elevation of [Ca2+]i; as a result, Ca2+ binds to cytosolic proteins which translate the changes in [Ca2+]i into activation of a number of key cellular functions. The purpose of this review is to provide a comprehensive description of the structural and functional characteristics of the various components of [Ca2+]i homeostasis in eukaryotes.

Phosphoinositides in frog skeletal muscle: a quantitative analysis

The contents of major phospholipids per g of wet wt. in frog skeletal muscle are: 5.3 mumol PC; 1.4 mumol PE; 1 mumol SM; 0.4 mumol PtdIns; 0.3 mumol CL; and 0.13 mumol PS. The quantities of polyphosphoinositides per g of wet wt. are: 181 nmol PtInsP; 28 nmol PtdInsP2; and 8 nmol lyso-PtdInsP2. The specific activity of labelling of the total muscle ATP attained by external incubation with [32P]Pi was found to be 57 dpm/nmol x g muscle wet wt. PtdInsP2, the highest labelled polyphosphoinositide, showed a specific activity of 64,000 dpm/nmol per g muscle wet wt., suggesting that high specific activity ATP may be compartmentalized in the local environment of the triads and used as a substrate by the PtdIns and PtInsP kinase in that region. PtdInsP2 which is the immediate precursor for the release of InsP3, is found at a significant concentration and strategically located for its postulated role as a substrate for the action of phosphoinositidase C. The presence of a novel endogenous polyphosphoinositide, lyso-PtdInsP2, in animal tissues is reported for the first time. Electrical stimulation leads towards a rapid catabolization of polyphosphoinositides revealed by reductions in the 3H- and 32P-labelling, suggesting that muscle excitation is associated with the activation of breaking down of polyphosphoinositides.

Does muscle activation occur by direct mechanical coupling of transverse tubules to sarcoplasmic reticulum?

Our knowledge of the physiological and biochemical constituents of skeletal muscle excitation has increased greatly during the last few years but this has not led to a consensus of the physiological mode of muscle activation. Three hypotheses of transmission, involving either transmitter-receptor interaction or direct mechanical coupling, are still under active consideration. The hypothesis of direct mechanical coupling currently being evaluated proposes that the dihydropyridine receptor in the transverse tubules serves as a voltage sensor that communicates directly with the junctional foot protein/Ca2+ channel of sarcoplasmic reticulum to initiate opening of the channel.

Triadic proteins of skeletal muscle

Biochemical approaches toward understanding the mechanism of muscle excitation have in recent years been directed to identification and isolation of proteins of the triad junction. The principal protein described--the junctional foot protein (JFP)2--was initially identified by morphological criteria and isolated using antibody-affinity chromatography. Subsequently this protein was described as the ryanodine receptor. It has been isolated and incorporated into lipid bilayers as a cation channel. This in its turn has directed attention toward the transverse (T)-tubular junctional constituents. Three approaches employing the JFP as a probe toward identifying these moieties on the T-tubule are described here. The binding of the JFP to the dihydropyridine receptor, which has been hypothesized to be the voltage sensor in excitation-contraction coupling, is also discussed. The detailed architecture and function of T-tubular proteins remain to be resolved.