Metalloendoprotease Cleavage of 18.2- and 14.1-Kilodalton Basic Proteins Dissociating from Rodent Myelin Membranes Generates 10.0- and 5.9-Kilodalton G-Terminal Fragments

Rat and guinea pig myelin membranes were incubated at physiological ionic strength with millimolar concentrations of Ca2+/Mg2+ ions (37 degrees C; pH 7.4). After 1-3 h, electrophoresis of the membranes revealed loss of 50% of 18.2- and 14.1-kilodalton (kDa) forms of myelin basic protein (MBP). Concomitantly, peptides representing 25% of the original membrane-associated MBP were detected in incubation media. Roughly equal amounts of MBP fragments with molecular masses of 10.0 and 8.4 kDa were found in media from guinea pig myelin incubations. Media from rat myelin experiments contained a major 8.4-kDa and minor 10.0- and 5.9-kDa MBP peptides. Kinetic studies implied that proteolysis occurred subsequent to MBP dissociation from the membranes. Immunoblotting studies indicated that both the 18.2- and 14.1-kDa forms of MBP were cleaved near residue 73 to produce a 10.0- and 5.9-kDa C-terminal fragment, respectively. Degradation of MBP in myelin membranes was partially inhibited by only 5-20% using leupeptin (20 microM) but up to 50% by dithiothreitol mM), phenylmethylsulphonyl fluoride (1 mM), and phosphoramidon (50 microM) but up to 50% by dithiothreitol (DDT, 10 mM). Only DDT and 1,10-phenanthroline substantially blocked the formation of the characteristic 10.0-and 5.9-kDa C-terminal fragments. This suggests that MBP, dissociating from myelin membrane preparations, is cleaved near residue 73 by a metalloendoprotease distinct from N-ethylmaleimide/leupeptin-sensitive calpains and phosphoramidon-sensitive endopeptidase 24.11.     

Zinc Ions Stabilise the Association of Basic Protein with Brain Myelin Membranes

Myelin basic protein (MBP) dissociated from brain myelin membranes when they were incubated (37 degrees C; pH 7.4) at physiological ionic strength. Zinc ions inhibited, and calcium promoted, this process. Protease activity in the membrane preparations cleaved the dissociated MBP into both small (less than 4 kilodaltons) and large (greater than 8 kilodaltons) fragments. The latter were detected, together with intact MBP, by gel electrophoresis of incubation media. Zinc ions appeared to act in two distinct processes. In the presence or absence of added CaCl2, zinc ions in the range 0.1-1 mM inhibited MBP-membrane dissociation. This process was relatively insensitive to heat and Zn2+ could be substituted by either copper (II) or cobalt (II) ions. A second effect was evident only in the presence of added calcium ions, when lower concentrations of Zn2+ (less than 0.1 mM) inhibited MBP-membrane dissociation and the accumulation of intact MBP in incubation media. This process was heat sensitive and only copper (II), but not cobalt (II), ions could replace Zn2+. To determine whether endogenous zinc in myelin membranes is bound to MBP, preparations were solubilised in buffers containing Triton X-100/2 mM CaCl2 and subjected to gel filtration. Endogenous zinc, as indicated by a dithizone-binding method, eluted with fractions containing both MBP and proteolipid protein (PLP). Thus, one means whereby zinc stabilises association of MBP with brain myelin membranes may be by promoting its binding to PLP.     

Analysis of the membrane-interacting domains of myelin basic protein by hydrophobic photolabeling

Myelin basic protein is a water soluble membrane protein which interacts with acidic lipids through some type of hydrophobic interaction in addition to electrostatic interactions. Here we show that it can be labeled from within the lipid bilayer when bound to acidic lipids with the hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (TID) and by two lipid photolabels. The latter included one with the reactive group near the apolar/polar interface and one with the reactive group linked to an acyl chain to position it deeper in the bilayer. The regions of the protein which interact hydrophobically with lipid to the greatest extent were determined by cleaving the TID-labeled myelin basic protein (MBP) with cathepsin D into peptides 1-43, 44-89, and 90-170. All three peptides from lipid-bound protein were labeled much more than peptides from the protein labeled in solution. However, the peptide labeling pattern was similar for both environments. The two peptides in the N-terminal half were labeled similarly and about twice as much as the C-terminal peptide indicating that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half. MBP can be modified post-translationally in vivo, including by deamidation, which may alter its interactions with lipid. However, deamidation had no effect on the TID labeling of MBP or on the labeling pattern of the cathepsin D peptides. The site of deamidation has been reported to be in the C-terminal half, and its lack of effect on hydrophobic interactions of MBP with lipid are consistent with the conclusion that the N-terminal half interacts hydrophobically more than the C-terminal half. Since other studies of the interaction of isolated N-terminal and C-terminal peptides with lipid also indicate that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half, these results from photolabeling of the intact protein suggest that the N-terminal half of the intact protein interacts with lipid in a similar way as the isolated peptide. The similar behavior of the intact protein to that of its isolated peptides suggests that when the purified protein binds to acidic lipids, it is in a conformation which allows both halves of the protein to interact independently with the lipid bilayer. That is, it does not form a hydrophobic domain made up from different parts of the protein.     

Extensive degradation of myelin basic protein isoforms by calpain following traumatic brain injury

Axonal injury is one of the key features of traumatic brain injury (TBI), yet little is known about the integrity of the myelin sheath. We report that the 21.5 and 18.5-kDa myelin basic protein (MBP) isoforms degrade into N-terminal fragments (of 10 and 8 kDa) in the ipsilateral hippocampus and cortex between 2 h and 3 days after controlled cortical impact (in a rat model of TBI), but exhibit no degradation contralaterally. Using N-terminal microsequencing and mass spectrometry, we identified a novel in vivo MBP cleavage site between Phe114 and Lys115. A MBP C-terminal fragment-specific antibody was then raised and shown to specifically detect MBP fragments in affected brain regions following TBI. In vitro naive brain lysate and purified MBP digestion showed that MBP is sensitive to calpain, producing the characteristic MBP fragments observed in TBI. We hypothesize that TBI-mediated axonal injury causes secondary structural damage to the adjacent myelin membrane, instigating MBP degradation. This could initiate myelin sheath instability and demyelination, which might further promote axonal vulnerability.     

Identification of Novel Site‐Specific Alterations in the Modification Level of Myelin Basic Protein Isolated from Mouse Brain at Different Ages Using Capillary Electrophoresis–Mass Spectrometry

Myelin basic protein (MBP) is a multifunctional protein involved in maintaining the stability and integrity of the myelin sheath by a variety of interactions with membranes and other proteins. MBP is subjected to extensive posttranslational modifications (PTMs) that are known to be crucial for the regulation of these interactions. Here, we report capillary electrophoresis–mass spectrometric (CE–MS) analysis for the separation and identification of MBP peptides that incorporate the same PTM at different sites, creating multiple localization variants, and the ability to analyze challenging modifications such as asparagine and glutamine deamidation, isomerization, and arginine citrullination. Moreover, we observed site‐specific alterations in the modification level of MBP purified from brain of mice of different age. In total, we identified 40 modifications at 33 different sites, which include both previously reported and seven novel modifications. The identified modifications include Nα‐terminal acetylation, mono‐ and dimethylation, phosphorylation, oxidation, deamidation, and citrullination. Notably, some new sites of arginine methylation overlap with the sites of citrullination. Our results highlight the need for sensitive and efficient techniques for a comprehensive analysis of PTMs.     

Interaction of two complementary fragments of the bovine spinal cord myelin basic protein with phospholipid bilayers. An ESR spin-label study

The myelin basic protein (MBP) from bovine spinal cord was cleaved at the single tryptophan residue to produce an N-terminal fragment (F1) of molecular weight 12.6K and a C-terminal fragment (F2) of molecular weight 5.8K. The interactions of the two fragments with bilayers of the acidic lipid dimyristoylphosphatidylglycerol (DMPG) were compared with those of the intact protein, by using both chemical binding assays and spin-label electron spin resonance spectroscopy. The saturation binding stoichiometries of the two fragments were found to sum to that of the MBP, having values of 11, 24, and 36 mol of DMPG/mol of protein for F2, F1, and the MBP, respectively. The strength of binding was found to increase in the order F2 less than F1 less than MBP, which follows that of the net charges on the different fragments. The ionic strength dependence of the protein binding indicated that the interaction is primarily of electrostatic origin. The efficiency of displacement of the proteins by salt was in the order F2 greater than F1 greater than MBP, which correlates with both the strength of binding and the net charge on the different protein fragments. Nitroxide derivatives of phosphatidylglycerol (PG) labeled on the sn-2 chain were used to probe the protein-induced changes in the acyl chain dynamics. Both the fragments and the MBP decreased the lipid chain mobility as recorded by the C-5 atom and C-12 atom position nitroxide-PG spin-labels, in a manner which followed the protein binding curves.(ABSTRACT TRUNCATED AT 250 WORDS)     

Monoclonal Antibodies to Human Myelin Basic Protein

SJL/J and (SJL X PL) F1 hybrid mice were immunized with intact human myelin basic protein (MBP) or the three major peptic fragments of MBP, residues 1-38, 39-89, and 90-170. Immune spleen cells were fused with mouse myeloma P3 X 63Ag8 (NS1) cells in the presence of polyethylene glycol. Hybridoma supernatant culture fluids were screened for antibody to MBP by a solid-phase radioimmunoassay (RIA). The specificity of the monoclonal antibody (mAb) was characterized by RIA using the three major MBP peptic fragments and subfragments as well as MBP and MBP fragments of different species with known amino acid sequence differences. Six MBP mAbs were generated, one of them IgM isotype and the remainder IgG isotypes. One mAb each reacted against regions of residues 22-38, 39-69, 70-89, 90-116, and two reacted against residues 118-157. Immunoblots also showed that the five IgG mAbs were reactive against MBP and the peptic fragment of MBP containing the epitope. Immunohistochemical studies showed the IgG mAbs specifically stained myelinated fiber tracts in human brain tissue.     

Preferential peptide specificity and HLA restriction of myelin basic protein-specific T cell clones derived from MS patients.

A panel of 17 myelin basic protein (MBP)-specific T lymphocyte clones were generated from four multiple sclerosis (MS) patients. All T cell clones expressed CD4 phenotype and 14 clones exhibited substantial cytotoxic activity on MBP-coated target cells. T cell recognition sites of the clones on human MBP were identified by using MBP fragments and synthetic peptides. Despite the fact that at least three epitopes were defined, these T cell clones displayed a striking bias to the C-terminal peptide 149-171 independent of differences in HLA-DR and DQ expression. In addition, the T cell responses of the clones appeared to be restricted by HLA-DR molecules irrespective of peptide specificities. The present study suggests an immunodominant property of the C-terminal peptide for HLA-DR-restricted T cell responses to MBP. However, its association with encephalitogenicity in humans and its potential pathologic importance in MS await further clarification.     

Is myelin basic protein crystallizable?

Myelin basic protein (MBP) is the predominant extrinsic protein in both central and peripheral nervous system myelins. It is thought to be involved in the stabilizing interactions between myelin membranes, and it may play an important role in demyelinating diseases such as multiple sclerosis. In spite of the fact that this abundant protein has been known for almost three decades, its three-dimensional crystal structure has not yet been determined. In this study we report on our extensive attempts to crystallize the major 18.5 kDa isoform of MBP. We used MBP having different degrees of purity, ranging from crude MBP (that was acid or salt extracted from isolated myelin), to highest purity single isoform. We used conventional strategies in our search for a suitable composition or a crystallization medium. We applied both full and incomplete factorial searches for crystallization conditions. We analyzed the available data on proteins which have previously resisted crystallization, and applied this information to our own experiments. Nevertheless, despite our efforts which included 4600 different conditions, we were unable to induce crystallization of MBP. Previous work on MBP indicates that when it is removed from its native environment in the myelin membrane and put in crystallization media, the protein adopts a random coil conformation and persists as a population of structurally non-identical molecules. This thermodynamically preferred state presumably hinders crystallization, because the most fundamental factor of protein crystallization-homogeneity of tertiary structure-is lacking. We conclude that as long as its random coil flexibility is not suppressed, 18.5 kDa MBP and possibly also its isoforms will remain preeminent examples of proteins that cannot be crystallized.     

Release of Proteins from the Surface of Bovine Central Nervous System Myelin by Salts and Phospholipases

Incubation of bovine CNS myelin with phospholipase C from Bacillus cereus under conditions that lead to extensive phospholipid degradation caused 10% of the myelin protein to be released from the membrane. The myelin basic protein (MBP) was a major component of the dissolved protein. Comparable incubations with phospholipase C from Clostridium perfringens, phosphatidylinositol-specific phospholipase C from Staphylococcus aureus, or cabbage phospholipase D removed little MBP. However, concentrations of sodium chloride near 1 M and concentrations of divalent metal ions between 50 and 600 mM released typically 9-12% of the total myelin protein, with MBP again as the predominant component. Repetitive washing with calcium chloride solutions resulted in dissolution of over 90% of the MBP. When myelin was incubated in 1.0 M sodium chloride or 25 mM calcium chloride, the MBP was cleaved largely into two major peptides with apparent molecular weights near 14,000 and 12,000, but with 200 mM or higher concentrations of calcium chloride most of this protein remained intact. With appropriate manipulation of the ionic milieu, it is thus possible to remove most of this extrinsic protein from the myelin surface under relatively mild conditions. The conditions that release the protein suggest that it is held at the membrane surface by ionic interactions.     

A Tale of Two Citrullines—Structural and Functional Aspects of Myelin Basic Protein Deimination in Health and Disease

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocyte membranes and is responsible for adhesion of these surfaces in the multilayered myelin sheath. The pattern of extensive post-translational modifications of MBP is dynamic during normal central nervous system (CNS) development and during myelin degeneration in multiple sclerosis (MS), affecting its interactions with the myelin membranes and with other molecules. In particular, the degree of deimination (or citrullination) of MBP is correlated with the severity of MS, and may represent a primary defect that precedes neurodegeneration due to autoimmune attack. That the degree of MBP deimination is also high in early CNS development indicates that this modification plays major physiological roles in myelin assembly. In this review, we describe the structural and functional consequences of MBP deimination in healthy and diseased myelin. Special issue dedicated to Drs. Anthony and Celia Campagnoni.     

ADP-ribosylation of myelin basic protein by cholera toxin.

Cholera toxin ADP-ribosylates four types of myelin basic proteins (MBPs) of Mr 14,000, 17,500, 19,000 and 22,000 in rat brain myelin. On an analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, MBP underwent mono- and multi-(ADP-ribosyl)ation by cholera toxin and thus modified MBP migrated on the gel as several discrete protein bands, the molecular masses of which were apparently larger by 500-2000 daltons than that of the corresponding untreated MBP. On average, 1.1 mol of ADP-ribosyl residue was incorporated into 1 mol of MBP. Four types of purified MBPs were also ADP-ribosylated by cholera toxin dependent on GTP and the protein factor for the ADP-ribosylation. The results show evidence that MBP is one of major and specific substrates of cholera toxin in brain membranes.     

Biochemical Analysis of Myelin Proteins in a Novel Neurological Mutant: The Taiep Rat

Abstract: Hemispheres, spinal cords, and sciatic nerves were taken from taiep, carrier, and control rats at ages ranging from 1 day to 16 months. Absolute myelin yields from CNS taiep tissues peaked at ～2 months and then decreased until they reached a low but stable level. Myelin yield from the affected hemispheres expressed as a percentage of age-matched controls decreased continuously from 2 weeks until it reached a stable level of ～10–15%. The same was true for the spinal cords, but here the myelin yield reached a plateau at a slightly higher percentage of 20–25%. In comparison with control rats, isolated CNS myelin fractions from the affected rats had a greater content of high molecular weight proteins. Western blot analyses of CNS homogenates revealed that myelin basic protein (MBP), proteolipid protein, and 2′,3′-cyclic nucleotide 3′-phosphodiesterase were all present but decreased to levels generally consistent with the deficiencies of myelin. However myelin-associated glycoprotein (MAG) levels always were reduced much more than those of the other three myelin proteins, and at younger ages the apparent molecular weight for MAG was increased in the mutants. Western blot analyses of sciatic nerve homogenates showed that the levels of MBP, MAG, and P₀ were not significantly different in control and mutant animals. These results suggested an early hypomyelination of the CNS, with peak levels of myelin at 2 months, followed by a prolonged period of myelin loss, until a very low but stable myelin level was reached. The consistently greater loss of MAG, in comparison with other CNS myelin proteins, is different from most other hypomyelinating mutants in which MAG is relatively preserved in comparison with the proteins of compact myelin. This might be due to microtubular abnormalities in the taiep mutant interfering with transport of myelin proteins and having the greatest effect on MAG because of its most distal location in the periaxonal oligodendroglial membranes.     

An analysis of the regions of the myelin basic protein that bind to phosphatidylcholine

The interactions of phosphatidylcholine (PC) to regions of the myelin basic protein (MBP) was examined. In solid phase binding assays the nature of the binding of unilamellar vesicles of¹⁴C-labeled phosphatidylcholine to bovine 18.5 kDa MBP, its N- and C-terminal peptide fragments, photooxidized 18.5 kDa MBP and the mouse 14 kDa protein, with an internal deletion of residues 117–157, was studied. The data were analyzed by computer-generated Scatchard plots in which non-specific binding was eliminated. Non-cooperative, low affinity binding of PC vesicles to MBP was observed, and this binding found to be sensitive to pH and ionic changes. At an ionic strength of 0.1 and pH 7.4, the binding of PC to the 14 kDa mouse MBP exhibited a K_d similar to that obtained with both the N-terminal and photooxidized 18.5 kDa bovine MBP. The studies indicated that the sites of PC interaction with MBP are located in the N-terminal region of the protein. The C-terminal region appeared to modulate the strength of the interaction slightly. Under similar conditions, lysozyme did not bind PC liposomes, and histone bound them nonspecifically.     

Epigenetic factors up-regulate expression of myelin proteins in the dysmyelinating jimpy mutant mouse

Proteolipid protein (PLP) is a major structural component of central nervous system (CNS) myelin. Evidence exists that PLP or the related splice variant DM-20 protein may also play a role in early development of oligodendrocytes (OLs), the cells that form CNS myelin. There are several naturally occurring mutations of the PLP gene that have been used to study the roles of PLP both in myelination and in OL differentiation. The PLP mutation in the jimpy (jp) mouse has been extensively characterized. These mutants produce no detectable PLP and exhibit an almost total lack of CNS myelin. Additionally, most OLs in affected animals die prematurely, before producing myelin sheaths. We have studied cultures of jp CNS in order to understand whether OL survival and myelin formation require production of normal PLP. When grown in primary cultures, jp OLs mimic the relatively undifferentiated phenotype of jp OLs in vivo. They produce little myelin basic protein (MBP), never immunostain for PLP, and rarely elaborate myelin-like membranes. We report here that jp OLs grown in medium conditioned by normal astrocytes synthesize MBP and incorporate it into membrane expansions. Some jp OLs grown in this way stain with PLP antibodies, including an antibody to a peptide sequence specific for the mutant jp PLP. This study shows that: (1) an absence of PLP does not necessarily lead to dysmyelination or OL death; (2) OLs are capable of translating at least a portion of the predicted jp PLP; (3) the abnormal PLP made in the cultured jp cells is not toxic to OLs. These results also highlight the importance of environmental factors in controlling OL phenotype. © 1996 John Wiley & Sons, Inc.     

Classic 18.5- and 21.5-kDa Myelin Basic Protein Isoforms Associate with Cytoskeletal and SH3-Domain Proteins in the Immortalized N19-Oligodendroglial Cell Line Stimulated by Phorbol Ester and IGF-1

The 18.5-kDa classic myelin basic protein (MBP) is an intrinsically disordered protein arising from the Golli (Genes of Oligodendrocyte Lineage) gene complex and is responsible for compaction of the myelin sheath in the central nervous system. This MBP splice isoform also has a plethora of post-translational modifications including phosphorylation, deimination, methylation, and deamidation, that reduce its overall net charge and alter its protein and lipid associations within oligodendrocytes (OLGs). It was originally thought that MBP was simply a structural component of myelin; however, additional investigations have demonstrated that MBP is multi-functional, having numerous protein-protein interactions with Ca²⁺-calmodulin, actin, tubulin, and proteins with SH3-domains, and it can tether these proteins to a lipid membrane in vitro. Here, we have examined cytoskeletal interactions of classic 18.5-kDa MBP, in vivo, using early developmental N19-OLGs transfected with fluorescently-tagged MBP, actin, tubulin, and zonula occludens 1 (ZO-1). We show that MBP redistributes to distinct ‘membrane-ruffled’ regions of the plasma membrane where it co-localizes with actin and tubulin, and with the SH3-domain-containing proteins cortactin and ZO-1, when stimulated with PMA, a potent activator of the protein kinase C pathway. Moreover, using phospho-specific antibody staining, we show an increase in phosphorylated Thr98 MBP (human sequence numbering) in membrane-ruffled OLGs. Previously, Thr98 phosphorylation of MBP has been shown to affect its conformation, interactions with other proteins, and tethering of other proteins to the membrane in vitro. Here, MBP and actin were also co-localized in new focal adhesion contacts induced by IGF-1 stimulation in cells grown on laminin-2. This study supports a role for classic MBP isoforms in cytoskeletal and other protein-protein interactions during membrane and cytoskeletal remodeling in OLGs.     

Cloned proteolipid protein and myelin basic protein cDNA. Transcription of the two genes during myelination

cDNA clones of rat brain proteolipid protein (PLP), also named lipophilin, the major integral myelin membrane protein, and of myelin basic protein (MBP), the major extrinsic myelin protein, have been isolated from a rat brain cDNA library cloned into the PstI site of pBR322. Poly(A)+ RNA from actively myelinating 18-day-old rats has been reversely transcribed. Oligonucleotides synthesized according to the established amino-acid sequence of lipophilin and the nucleotide sequence of the small myelin basic protein of the N-terminal, the central and C-terminal region of their sequences were used as hybridization probes for screening. The largest insert in one of several lipophilin clones was 2,585 base pairs (bp) in length (pLp 1). It contained 521 bp of the C-terminal coding sequence and the complete 2,064 bp long non-coding 3' sequence. The myelin basic protein cDNA insert of clones pMBP5 and pMBP6 is 2,530 bp long and that of clones pMBP2 and pMBP3 640 bp. These clones were also characterized. pMBP2 was sequenced and used together with the lipophilin cDNA clones as hybridization probes to estimate the lipophilin and myelin basic protein mRNA levels of rat brain during the myelination period. The expression of the lipophilin and myelin basic protein genes during development of the myelin sheath appears to be strictly coordinated.     

Acid endopeptidase activity of human myelin, elicited by using exogenous myelin basic protein as enzyme substrate

Purified human myelin was incubated with exogenous myelin basic protein (MBP) at pH 4.0 to see if there is acid proteinase activity associated with myelin. Following incubation for 12 h up to 70% of MBP was degraded. On electrophoresis peptide fragments of MBP between 15.8 and 9.4 kDa were consistent with an endopeptic cleavage of MBP. Unlike the exogenous substrate MBP associated with myelin was only slightly degraded under the experimental conditions used. The results show that proteinase activity associated with isolated myelin may be elicited and further evaluated by using MBP as enzyme substrate.     

Structured functional domains of myelin basic protein: cross talk between actin polymerization and Ca(2+)-dependent calmodulin interaction

The 18.5-kDa myelin basic protein (MBP), the most abundant isoform in human adult myelin, is a multifunctional, intrinsically disordered protein that maintains compact assembly of the sheath. Solution NMR spectroscopy and a hydrophobic moment analysis of MBP's amino-acid sequence have previously revealed three regions with high propensity to form strongly amphipathic α-helices. These regions, located in the central, N- and C-terminal parts of the protein, have been shown to play a role in the interactions of MBP with cytoskeletal proteins, Src homology 3-domain-containing proteins, Ca(2+)-activated calmodulin (Ca(2+)-CaM), and myelin-mimetic membrane bilayers. Here, we have further characterized the structure-function relationship of these three domains. We constructed three recombinant peptides derived from the 18.5-kDa murine MBP: (A22-K56), (S72-S107), and (S133-S159) (which are denoted α1, α2, and α3, respectively). We used a variety of biophysical methods (circular dichroism spectroscopy, isothermal titration calorimetry, transmission electron microscopy, fluorimetry, and solution NMR spectroscopy and chemical shift index analysis) to characterize the interactions of these peptides with actin and Ca(2+)-CaM. Our results show that all three peptides can adopt α-helical structure inherently even in aqueous solution. Both α1- and α3-peptides showed strong binding with Ca(2+)-CaM, and both adopted an α-helical conformation upon interaction, but the binding of the α3-peptide appeared to be more dynamic. Only the α1-peptide exhibited actin polymerization and bundling activity, and the addition of Ca(2+)-CaM resulted in depolymerization of actin that had been polymerized by α1. The results of this study proved that there is an N-terminal binding domain in MBP for Ca(2+)-CaM (in addition to the primary site located in the C-terminus), and that it is sufficient for CaM-induced actin depolymerization. These three domains of MBP represent molecular recognition fragments with multiple roles in both membrane- and protein-association.