Squid neurofilaments. Phosphorylation and Ca2+-dependent proteolysis in situ.

Three major polypeptides co-purify with neurofilaments from squid (Loligo forbesi) axoplasm: P60 (apparent Mr 60,000), P200 (apparent Mr 200,000) and Band 1 (apparent Mr 400,000). Anti-IFA, a monoclonal antibody that recognizes an epitope common to all classes of intermediate filaments, binds to P200 and P60. When axoplasm is incubated with [32P]Pi, the major phosphorylated polypeptides are P200 and Band 1. We have investigated Ca2+-dependent proteolysis of [32P]phosphorylated axoplasm in order to localize the major sites of phosphorylation and to probe the arrangement of the polypeptides in the filament. The proteinase preferentially cleaves P200 and Band 1, liberating the phosphorylated domains. Analysis of proteolysed filaments by electron microscopy and gel electrophoresis shows that most of P200 and Band 1 can be cleaved while still maintaining intact filaments. We suggest that P200 is initially cleaved within a single highly sensitive region, generating two major fragments called P100p (apparent Mr 100,000) and P110s (apparent Mr 110,000). P100p contains the Anti-IFA epitope and co-sediments with filaments, whereas P110s is highly phosphorylated and does not sediment with filaments. Band 1 is cleaved to produce a soluble high-Mr fragment that is phosphorylated and that represents a major portion of the undigested component, whereas P60 is relatively resistant to limited proteolysis. Thus proteolysis appears to define two major filament domains: a conserved core that forms the backbone of the filament, and a highly phosphorylated peripheral region that is not essential for filament integrity.     

Neurofilament protein is phosphorylated in the squid giant axon

We have observed the phosphorylation of neurofilament protein from squid axoplasm. Phosphorylation is demonstrated by 32P labeling of protein during incubation of axoplasm with [gamma-32P]ATP. When the labeled proteins are separated by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), two bands, at 2.0 x 10(5) daltons and greater than 4 x 10(5) daltons, contain the bulk of the 32P. The 2.0 x 10(5)-dalton phosphorylated polypeptide comigrates on SDS-PAGE with one of the subunits of squid neurofilament protein. Both major phosphorylated polypeptides co-fractionate with neurofilaments in discontinuous sucrose gradient centrifugation and on gel filtration chromatography on Sepharose 4B. The protein-phosphate bond behaves like a phospho-ester, and labeled phospho-serine is identified in an acid hydrolysate of the protein. The generality of this phenomenon in various species and its possible physiological significance are discussed.     

Dephosphorylation of neurofilament proteins enhances their susceptibility to degradation by calpain.

The degradation of phosphorylated and dephosphorylated neurofilament proteins by the Ca2+-activated neutral proteinase calpain was studied. Neurofilaments were isolated from bovine spinal cord, dephosphorylated by alkaline phosphatase (from Escherichia coli) and radioiodinated with [125I]-Bolton-Hunter reagent. The radioiodinated neurofilament proteins (untreated and dephosphorylated) were incubated in the presence and absence of calpain from rabbit skeletal muscle, and the degradation rates of large (NF-H), mid-sized (NF-M) and small (NF-L) neurofilament polypeptides were analysed by SDS/polyacrylamide-gel electrophoresis and autoradiography. The degradation of dephosphorylated neurofilament proteins occurred at a higher rate, and to a greater extent, than did that of the phosphorylated (untreated) neurofilament proteins. The dephosphorylated high-molecular-mass neurofilament (NF-HD) was proteolyzed 6 times more quickly than the untreated NF-H. The degradation rate of the NF-M and NF-L neurofilament proteins was also enhanced after dephosphorylation, but less than that of NF-H. This indicates that the dephosphorylation of neurofilament proteins can increase their sensitivity to calpain degradation.     

Chicken skeletal muscle has three Ca2+-dependent proteinases

Chicken breast muscle has three Ca2+-dependent proteinases, two requiring millimolar Ca2+ (m-calpain and high m-calpain) and one requiring micromolar Ca2+ (mu-calpain). High m-calpain co-purifies with mu-calpain through successive DEAE-cellulose (steep gradient), phenyl-Sepharose, octylamine agarose, and Sephacryl S-300 columns, but elutes after mu-calpain when using a shallow KCl gradient to elute a DEAE-cellulose column. The mu- and m-calpains have 80 and 28 kDa polypeptides and are analogous to the mu- and m-calpains that have been purified from bovine, porcine and rabbit skeletal muscle. High m-calpain, which seems to be a new Ca2+-dependent proteinase, is still heterogeneous after the DEAE-cellulose column eluted with a shallow KCl gradient. Additional purification through two successive HPLC-DEAE columns and one HPLC-SW-4000 gel permeation column produces a fraction having six major polypeptides and 6-8 minor polypeptides on SDS-PAGE. A 74-76 kDa polypeptide in this fraction reacts in Western blots with monospecific, polyclonal anti-calpain antibodies that react with both the 80 kDa and the 28 kDa polypeptides of mu- or m-calpain. High m-calpain also is related to mu- and m-calpain in that it causes the same limited digestion of skeletal muscle myofibrils, has a similar pH optimum near pH 7.9-8.4, requires Ca2+ for activity, and reacts with the calpain inhibitor, calpastatin, and a variety of serine and cysteine proteinase inhibitors in a manner identical to mu- and m-calpain. High m-calpain differs from mu- and m-calpain in its elution off DEAE-cellulose columns and its requirement of 3800 microM Ca2+ for one-half maximal activity compared with 5.35 microM Ca2+ for mu-calpain and 420 microM Ca2+ for m-calpain. The physiological significance of high m-calpain in unclear. The presence of mu-calpain in chicken breast muscle suggests that all skeletal muscles contain both mu- and m-calpain, although the relative proportions of these two proteinases may vary in different species.     

The polypeptide composition of axoplasm and of neurofilaments from the marine worm Myxicola infundibulum.

1. Axoplasm from Myxicola contains two major polypeptides associated with neurofilaments, together with actin, tubulin and many minor polypeptide components. 2. Some of the minor polypeptides with molecular weights between 140,000 and 50,000 purify with neurofilaments under a variety of conditions and they appear to represent an integral part of the filament structure. 3. Peptide fingerprinting shows that the two major neurofilament polypeptides are almost identical. The fingerprint patterns from these major polypeptides share features with those obtained from the minor components. 4. Peptide fingerprinting has enabled us to propose a scheme for the main sites at which papain cleaves the major neurofilament polypeptides. In addition fingerprinting indicates how the minor components are related to the major polypeptides. 5. It is suggested that many of the minor neurofilament polypeptides could arise by proteolysis in vivo.     

Autophosphorylation of Calmodulin-Kinase II in Synaptic Junctions Modulates Endogenous Kinase Activity

Previous studies have purified from brain a Ca2+/calmodulin-dependent protein kinase II (designated CaM-kinase II) that phosphorylates synapsin I, a synaptic vesicle-associated phosphoprotein. CaM-kinase II is composed of a major Mr 50K polypeptide and a minor Mr 60K polypeptide; both bind calmodulin and are phosphorylated in a Ca2+/calmodulin-dependent manner. Recent studies have demonstrated that the 50K component of CaM-kinase II and the major postsynaptic density protein (mPSDp) in brain synaptic junctions (SJs) are virtually identical and that the CaM-kinase II and SJ 60K polypeptides are highly related. In the present study the photoaffinity analog [alpha-32P]8-azido-ATP was used to demonstrate that the 60K and 50K polypeptides of SJ-associated CaM-kinase II each bind ATP in the presence of Ca2+ plus calmodulin. This result is consistent with the observation that these proteins are phosphorylated in a Ca2+/calmodulin-dependent manner. Experiments using 32P-labeled peptides obtained by limited proteolysis of 60K and 50K polypeptides from SJs demonstrated that within each kinase polypeptide the same peptide regions contain both autophosphorylation and 125I-calmodulin binding sites. These results suggested that the autophosphorylation of CaM-kinase II could regulate its capacity to bind calmodulin and, thus, its capacity to phosphorylate substrate proteins. By using 125I-calmodulin overlay techniques and sodium dodecyl sulfate-polyacrylamide gel electrophoresis we found that phosphorylated 50K and 60K CaM-kinase II polypeptides bound more calmodulin (50-70%) than did unphosphorylated kinase polypeptides. Levels of in vitro CaM-kinase II activity in SJs were measured by phosphorylation of exogenous synapsin I. SJs containing highly phosphorylated CaM-kinase II displayed greater activity in phosphorylating synapsin I (300% at 15 nM calmodulin) relative to control SJs that contained unphosphorylated CaM-kinase II. The CaM-kinase II activity in phosphorylated SJs was indistinguishable from control SJs at saturating calmodulin concentrations (300-1,000 nM). These findings show that the degree of autophosphorylation of CaM-kinase II in brain SJs modulates its in vitro activity at low and possibly physiological calmodulin concentrations; such a process may represent a mechanism of regulating this kinase's activity at CNS synapses in situ.     

Characterization of a cyclic nucleotide- and calcium-independent neurofilament protein kinase activity in axoplasm from the squid giant axon

The phosphorylation activity associated with a neurofilament-enriched cytoskeletal preparation isolated from the squid giant axon has been studied and compared to the phosphorylation activities in intact squid axoplasm. The high molecular weight (greater than 300 kDa) and 220-kDa neurofilament proteins are the major endogenous substrates for the kinases in the axoplasm and the neurofilament preparation, whereas 95- and less than 60-kDa proteins are the major phosphoproteins in the ganglion cell preparation. The squid axon neurofilament (SANF) protein kinase activity appeared to be both cAMP and Ca2+ independent and could phosphorylate both casein (Km = 40 microM) and histone (Km = 180 microM). The SANF protein kinase could utilize either ATP or GTP in the phosphotransferase reaction, with a Km for ATP of 58 microM and 129.4 microM for GTP when casein was used as the exogenous substrate; and 25 and 98.1 microM for ATP and GTP, respectively, when the endogenous neurofilament proteins were used as substrates. The SANF protein kinase activity was only slightly inhibited by 2,3-diphosphoglycerate and various polyamines at high concentrations and was poorly inhibited by heparin (34% inhibition at 100 micrograms/ml). The failures of heparin to significantly inhibit and the polyamines to stimulate the SANF protein kinase indicate that it is not a casein type II kinase. The relative efficacy of GTP as a phosphate donor indicates that SANF protein kinase differs from known casein type I kinases. Phosphorylated (32P-labeled) neurofilament proteins were only slightly dephosphorylated in the presence of axoplasm or stellate ganglion cell supernatants, and the neurofilament-enriched preparation did not dephosphorylate 32P-labeled neurofilament proteins. The axoplasm and neurofilament preparations had no detectable protein kinase inhibitor activity, but a strong inhibitor activity, which was not dialyzable but was heat inactivatable, was found in ganglion cells. This inhibitor activity may account for the low phosphorylation activity found in the stellate ganglion cells and may indicate inhibitory regulation of SANF protein kinase activity in the ganglion cell bodies.     

Purification of a calcium-activated neutral proteinase from bovine brain

A calcium-activated neutral proteinase (CANP) resolved into three components has been partially purified from bovine brain. The method of isolation has resulted in 22,000, 7,100, and 8,000-fold purification for CANP I, II and III respectively. All three fractions require Ca2+ for activation. The characterization of the purified CANP I has shown that it is activated by 250 microM Ca2+ and the enzyme loses its activity when incubated in the presence of Ca2+ without substrate. Mg2+ is ineffective. The enzyme degrades neurofilament triplet proteins, tubulin and casein efficiently. The myelin basic protein is hydrolyzed after longer incubation. Bovine serum albumin and histones are unaffected. The enzyme is active at pH 5.5 to 9.0 with optimum between pH 7.5 and 8.5. It has a Km of 1.8 X 10(-7) M for the 69,000 dalton neurofilament protein. The enzyme is inhibited by sulphydryl blocking reagents and also by EGTA, leupeptin and E-64c. The SDS-PAGE analysis of the enzyme fractions has shown a major band at 66-68,000 daltons and two minor bands at 60,000 and 48-50,000 daltons for CANP I; a major band at 48-50,000 daltons and a minor band at 30-32,000 daltons for CANP II and a predominant doublet at 30-32,000 daltons with a minor band at 48-50,000 daltons for CANP III. The degradation of neurofilament proteins suggests that the CANP(s) may be involved in the turnover of these proteins.     

Subunit structure and multiple phosphorylation sites of phospholamban

The phosphorylation-induced mobility shift of the high molecular weight form of phospholamban (24,500 daltons) in the cardiac sarcoplasmic reticulum produced on 3',5'-cyclic AMP (cAMP)-dependent phosphorylation with 5 mM ATP was resolved into five clear steps on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and on Ca2+-calmodulin-dependent phosphorylation into ten steps. The mobility shift of the low molecular weight form of phospholamban (less than 14,400 daltons) in these reactions occurred in one step and two steps, respectively. With the two protein kinase activities, the electrophoretic pattern of the mobility shifts of the high and low molecular weight forms of phospholamban was similar to that obtained with Ca2+-calmodulin-dependent protein kinase alone. The results of pulse-chase experiments involving the centrifuge column method suggested that the site(s) of phosphorylation by cAMP- and Ca2+-calmodulin-dependent protein kinase activities are on the same phospholamban molecule. Two-dimensional tryptic peptide maps of phosphorylated phospholamban indicated that cAMP-dependent protein kinase phosphorylates at a single site, A, and Ca2+-calmodulin-dependent protein kinase phosphorylates at sites C1 and C2 in the low molecular weight form, where A is different from C1 but may be the same as C2. The high molecular weight form of phospholamban is suggested to be a pentamer of identical monomers (low molecular weight form) having one phosphorylation site for cAMP-dependent protein kinase and two for Ca2+-calmodulin-dependent protein kinase.     

Use of site-directed mutations in the individual Ca2(+)-binding sites of calmodulin to examine Ca2(+)-induced conformational changes

Mutant versions of the calmodulin of Drosophila melanogaster have been prepared for use in the study of Ca2+ binding and Ca2(+)-induced conformational changes. In each mutant, a conserved glutamic acid residue indicated to play a critical role in Ca2+ binding has been mutated to glutamine in one of the Ca2(+)-binding sites. Thus a series of four proteins, each with an analogous mutation in one of the four binding sites, has been generated. Here the Ca2(+)-induced conformational changes in these proteins have been examined by use of the fluorescent hydrophobic reporter molecule, 9-anthroyl choline. These studies confirm earlier work which indicates that the carboxyl-terminal pair of Ca2(+)-binding sites shows cooperative Ca2+ binding to produce a major conformational change in the protein. However, these studies provide evidence that the sites of the amino-terminal pair are more independent in their Ca2+ binding properties and contribute individually to the conformational changes associated with Ca2+ binding in the amino-terminal half of the protein. This work also indicates that mutation of either of the amino-terminal Ca2(+)-binding sites can influence the conformational change produced by Ca2+ binding to the carboxyl-terminal sites.     

Calcium ion stimulated endogenous protein kinase catalyzed phosphorylation of peripheral nerve myelin proteins

Myelin isolated from the rat peripheral nervous system (sciatic nerve and cauda equina) contained Mg2+-dependent protein kinase that phosphorylated myelin polypeptides. Ca2+, in micromolar concentrations, markedly stimulated phosphorylation (half-maximal stimulation at 5 microM (free) Ca2+) but at higher concentrations (greater than 100 microM Ca2+) it caused inhibition. In the presence of Triton X-100, phosphorylation (+/-Ca2+) of myelin was increased and Ca2+ caused up to a 10-fold increase in phosphorylation. Among the myelin polypeptides, P0 (Mr, 28 000), a major glycoprotein, accounted for nearly 60% of the total phosphate incorporated into the myelin and Ca2+ markedly promoted phosphorylation of P0. Phosphorylation of other myelin polypeptides, P2 (Mr, 16 000), Y (Mr, 26 000), and P1 (Mr, 20 000), and the Ca2+-stimulatory effect on phosphorylation of these were also evident. Cyclic AMP (or other cyclic nucleotides) failed to show any significant stimulatory effect on myelin phosphorylation.     

Autolysis of the millimolar Ca2+-requiring form of the Ca2+-dependent proteinase from chicken skeletal muscle

The millimolar Ca2+-requiring form of the Ca2+-dependent proteinase from chicken breast skeletal muscle contains two subunit polypeptides of 80 and 28 kDa, just as the analogous forms of this proteinase from other tissues do. Incubation with Ca2+ at pH 7.5 causes rapid autolysis of the 80-kDa polypeptide to 77 kDa and of the 28-kDa polypeptide to 18 kDa. Autolysis of the 28-kDa polypeptide is slightly faster than autolysis of the 80-kDa polypeptide and is 90-95% complete after 10 s at 0 degrees C. Autolysis for 15 s at 0 degrees C converts the proteinase from a form requiring 250-300 microM Ca2+ to one requiring 9-10 microM Ca2+ for half-maximal activity, without changing its specific activity. The autolyzed proteinase has a slightly lower pH optimum (7.7 vs. 8.1) than the unautolyzed proteinase. The autolyzed proteinase is not detected in tissue extracts made immediately after death; therefore, the millimolar Ca2+-requiring proteinase is largely, if not entirely, in the unautolyzed form in situ.     

Sequence analysis of phospholamban. Identification of phosphorylation sites and two major structural domains

Phospholamban is a regulatory protein in cardiac sarcoplasmic reticulum that is phosphorylated by cAMP- and Ca2+/calmodulin-dependent protein kinase activities. In this report, we present the partial amino acid sequence of canine cardiac phospholamban and the identification of the sites phosphorylated by these two protein kinases. Gas-phase protein sequencing was used to identify 20 NH2-terminal residues. Overlap peptides produced by trypsin or papain digestion extended the sequence 16 residues to give the following primary structure: Ser-Ala-Ile-Arg-Arg-Ala-Ser-Thr-Ile-Glu-Met-Pro-Gln-Gln-Ala- Arg-Gln-Asn-Leu-Gln-Asn-Leu-Phe-Ile-Asn-Phe-(Cys)-Leu-Ile-Leu-Ile-(Cys)- Leu-Leu-Leu-Ile-. Phospholamban phosphorylated by either cAMP-dependent or Ca2+/calmodulin-dependent protein kinase was cleaved with trypsin, and the major phosphorylated peptide (comprising greater than 70% of the incorporated 32P label) was purified by reverse-phase high performance liquid chromatography. The identical sequence was revealed for the radioactive peptide obtained from phospholamban phosphorylated by either kinase: Arg-Ala-Ser-Thr-Ile-Glu-Met-Pro-Gln-Gln-. The adjacent residues Ser7 and Thr8 of phospholamban were identified as the unique sites phosphorylated by cAMP- and Ca2+/calmodulin-dependent protein kinases, respectively. These results establish that phospholamban is an oligomer of small, identical polypeptide chains. A hydrophilic, cytoplasmically oriented NH2-terminal domain on each monomer contains the unique, adjacent residues phosphorylated by cAMP- and Ca2+/calmodulin-dependent protein kinase activities. Analysis by hydropathic profiling and secondary structure prediction suggests that phospholamban monomers also contain a hydrophobic domain, which could form amphipathic helices sufficiently long to traverse the sarcoplasmic reticulum membrane. A model of phospholamban as a pentamer is presented in which the amphipathic alpha-helix of each monomer is a subunit of the pentameric membrane-anchored domain, which is comprised of an exterior hydrophobic surface and an interior hydrophilic region containing polar side chains.     

Uptake and release of 45Ca by Myxicola axoplasm

The binding and release of 45Ca by axoplasm isolated from Myxicola giant axons were examined. Two distinct components of binding were observed, one requiring ATP and one not requiring ATP. The ATP- dependent binding was largely prevented by the addition of mitochondrial inhibitors, whereas the ATP-independent component was unaffected by these inhibitors. The ATP-independent binding accounted for roughly two-thirds of the total 45Ca uptake in solutions containing an ionized [Ca2+] = 0.54 microM and was the major focus of this investigation. This fraction of bound 45Ca was released from the axoplasm at a rate that increased with increasing concentrations of Ca2+ in the incubation fluid. The ions Cd2+ and Mn2+ were also able to increase 45Ca efflux from the sample, but Co2+, Ni2+, Mg2+, and Ba2+ had no effect. The concentration-response curves relating the 45Ca efflux rate coefficients to the concentration of Ca2+, Cd2+, and Mn2+ in the bathing solution were S-shaped. The maximum rate of efflux elicited by one of these divalent ions could not be exceeded by adding a saturating concentration of a second ion. Increasing EGTA concentration in the bath medium from 100 to 200 microM did not increase 45Ca efflux; yet increasing the concentration of the EGTA buffer in the uptake medium from 100 to 200 microM and keeping ionized Ca2+ constant caused more 45Ca to be bound by the axoplasm. These results suggest the existence of high-affinity, ATP-independent binding sites for 45Ca in Myxicola axoplasm that compete favorably with 100 microM EGTA. The 45Ca efflux results are interpreted in terms of endogenous sites that interact with Ca2+, Cd2+, or Mn2+.     

Native (Na-+ + K-+)-dependent adenosine triphosphatase has two trypsin-sensitive sites.

Sodium and potassium adenosine triphosphatase ((Na + K)-ATPase) consists of two polypeptides, a large molecular weight polypeptide (MW 84,000 to 102,000) and a sialoglycoprotein (MW 35,000 to 57,000). Trypsin treatment of this complex selectively cleaves the large polypeptide into two fragments with molecular weights of 62,000 and 43,000. Simultaneously with the appearance of these fragments, (Na + K)-APTase activity is destroyed. Trypsin treatment of phosphorylated enzyme shows that he 43,000 molecular weight fragment is phosphorylated. If (Na + K)-ATPase is digested with trypsin in the presence of ATP, a 90,000 molecular weight fragment is produced. Disappearance of the large polypeptide, and loss of ATPase activity parallel the production of this fragment. Addition of strophanthidin to this mixture significantly lowers the amount of the 90,000 molecular weight fragment produced. Experiments on (Na + K)-ATPase of the red cell membrane suggest that trypsin is cleaving (Na + K)-ATPase at the interior surface of the plasma membrane.     

A CALCIUM ACTIVATED PROTEASE IN SQUID AXOPLASM

Evidence for a protease in squid axoplasm which is selectively activated by Ca²⁺ and blocked by SH-inhibitors is presented. This protease appears to be particularly effective in degrading squid neurofilament proteins, but also extensively degrades various other major protein components in axoplasm.     

Dynamics of Phosphorylation and Assembly of the High Molecular Weight Neurofilament Subunit in NB2a/d1 Neuroblastoma

In neuronal systems thus far studied, newly synthesized neurofilament subunits rapidly associate with the Triton-insoluble cytoskeleton and subsequently undergo extensive phosphorylation. However, in the present study we demonstrate by biochemical and immunological criteria that NB2a/d1 neuroblastoma cells also contain Triton-soluble, extensively phosphorylated 200-kDa high molecular weight neurofilament subunits (NF-H). High-speed centrifugation (100,000 g) of the Triton-soluble fraction for 1 h sedimented some, but not all, soluble NF-H subunits; immunoelectron microscopic analyses of the resulting pellet indicated that a portion of the NF-H subunits in this fraction are assembled into (Triton-soluble) neurofilaments. When cells were pulse labeled for 15 min with [35S]methionine, radiolabel was first associated with the Triton-soluble 200-kDa NF-H variants. Because only extensively phosphorylated NF-H subunits migrate at 200 kDa, whereas hypophosphorylated subunits migrate instead at 160 kDa, these findings suggest that some newly synthesized subunits were phosphorylated before they polymerized. In pulse-chase analyses, radiolabeled 200-kDa NF-H migrated into the 100,000 g particulate fraction of Triton-soluble extracts before its arrival in the Triton-insoluble cytoskeleton. Undifferentiated cells, which do not possess axonal neurites and lack a significant amount of Triton-insoluble, extensively phosphorylated NF-H, contain a sizeable pool of Triton-soluble extensively phosphorylated NF-H subunits and polymers. We interpret these data to indicate that the integration of newly synthesized NF-H into the cytoskeleton occurs in a progression of distinct stages, and that assembly of NF-H into neurofilaments and integration into the Triton-insoluble cytoskeleton are not prerequisites for the incorporation of certain phosphate groups on these polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for the utilization of extracellular [γ-32P]ATP for the phosphorylation of intracellular proteins in the squid giant axon

Proteins in the squid giant axon were labeled with ³²P by in vitro incubation of isolated axoplasm with radioactive [γ-³²P]adenosine triphosphate (ATP) and separated by polyacrylamide sodium dodecyl sulfate gel electrophoresis. The two major phosphorylated regions on the gel had molecular weights of 400 000 and 200 000. These two peaks appear to be neurofilament proteins of squid axoplasm. The same set of proteins was phosphorylated in the axoplasm regardless of whether the [γ-³²P]ATP was applied in situ intracellularly or extracelarly. These results suggest that ATP in the extracellular space is, by some ATP-translocation mechanism, utilized in the process of intracellular phosphorylation. Measurements of the apparent influx of ATP across the squid axon membrane yielded results consistent with the view that ATP in the extracellular fluid could be transported into the axoplasm.     

Probing of the structural stability of vimentin and desmin-type intermediate filaments with Ca2+-activated proteinase, thrombin and lysine-specific endoproteinase Lys-C

Intermediate filaments (IFs) reconstituted from purified, delipidated vimentin and desmin as well as respective protofilaments were subjected to degradation by Ca2+-activated neutral thiol proteinase, thrombin and lysine-specific endoproteinase Lys-C, respectively. The breakdown products were analyzed by SDS-polyacrylamide gel electrophoresis and negative stain electron microscopy. While Ca2+-activated proteinase and thrombin caused rapid and complete degradation of IFs with kinetics not significantly different from those of the degradation of protofilaments, lysine-specific endoproteinase did not exert any electron microscopically detectable effect on filament structure. Although both types of subunit proteins were truncated at their non-alpha-helical, C-terminal polypeptides by this proteinase, they were still able to assemble into 10 nm filaments. Closer electron microscopic inspection of IFs treated with Ca2+-activated proteinase revealed numerous ruptures along the filaments already at very early stages of digestion. SDS-polyacrylamide gel electrophoresis of the processed filaments in conjunction with previous biochemical characterizations of the breakdown of protofilaments by Ca2+-activated proteinase showed that these inhomogeneities primarily arose from degradation of the arginine-rich, non-alpha-helical N-termini of the filament proteins. These findings demonstrate that, although the N-terminus of vimentin and desmin is essential for filament stability, it is still highly susceptible to proteolytic attack in particular and very likely to posttranslational modification in general. Such structural modifications of the N-termini of IF proteins might exert great influences on the intracellular distribution and molecular organization of IFs in various physiological and pathological conditions.     

Interactions of calcium and other metal ions with caldolysin, the thermostable proteinase from Thermus aquaticus strain T351.

Caldolysin, the extracellular proteinase from the extreme thermophile Thermus aquaticus strain T351, is stabilized by Ca2+. A variety of metal ions were able to substitute for Ca2+. Most were unable to confer as much stability as Ca2+, with the exception of the lanthanide ions, which increased the half-life at 95 degrees C from 1 h to more than 4 h. Results from a variety of separation methods indicated that caldolysin binds 6 Ca2+ ions/molecule of enzyme. The presence of non-linear Ca2+ titration plots, and the removal of 4 Ca2+ ions/molecule by treatment with a cationic ion-exchange gel suggested that caldolysin possesses at least two different types of Ca2+-binding sites, with different affinities. Average binding constants of the two types of binding sites were 2.8 X 10(4)M-1 (for the low-affinity sites) and 7.5 X 10(5) M-1 (for the high-affinity sites). The total Ca2+-binding free energy for caldolysin was shown to be greater than for either thermolysin or Bacillus subtilis neutral proteinase. It appears that the higher thermostability of caldolysin is due to the presence of 6 Ca2+ ions rather than 4 Ca2+ ions/molecule.