Myelin basic protein: Interaction with calmodulin and gangliosides

Abstract

The structural characteristics of myelin basic protein (MBP) involved in protein-protein and protein-lipid interactions were investigated. Rabbit MBP could bind calmodulin (CaM) in the presence of Ca2+ to form a complex that remained undissociated in 8 M urea. However, no tight complex formation was observed when the divalent cation was absent. These results suggest that MBP may contain a hydrophobic domain similar to those in the other well-characterized CaM-binding proteins. The stoichiometry of calmodulin binding to MBP was approximately 1:1. Prior limited proteolysis of MBP with trypsin abolished the formation of the MBP-CaM complex, indicating that the entire MBP polypeptide may be involved in the recognition of the hydrophobic clefts in CaM. MBP also formed tight complexes with gangliosides, but the presence of Ca2+ was not required. Binding of gangliosides to MBP-CaM complex released CaM from the complex. The ganglioside-binding sites in MBP were determined after trisecting the protein at two glutamic acid residues with Staphylococcus aureus V8 protease. Subsequent binding studies revealed that a 9.5-kDa polypeptide, which may correspond to the NH2-terminal domain (residues 1-83) of MBP, had higher affinity for the binding of lucifer yellow CH-labeled GM1 than did the other two polypeptides, of apparent molecular mass (Mr) 5,500 and 4,500, respectively. Among the various proteins in purified guinea pig brain myelin, synaptosomes, and synaptosomal membranes, MBP was found to have the highest affinity in binding lucifer yellow CH-GM1.

Characterization of a Recombinant Murine 18.5-kDa Myelin Basic Protein

“…To emulate an amphipathic myelin-like environment, CD was performed in the presence of monosialoganglioside G M1 , which binds strongly to MBP and which has been shown to increase its proportion of ordered secondary structure significantly (48,85,86). Here, the same effect was observed for both proteins (Fig.…”

“…Monosialoganglioside G M1 and MBP interact very strongly and are present in roughly equimolar amounts in the myelin sheath in vivo (24,48). Thus, G M1 was used to emulate a myelin-like environment.…”

Three-dimensional Structure of Myelin Basic Protein

“…It also interacts with other proteins such as proteolipid protein (67,68), calmodulin (12), actin (51), and tubulin (52) and sequesters zinc (72). Thus, it is not unexpected that MBP forms extensive clusters on negatively charged lipid monolayers, especially in the low salt buffer G (Fig.…”

“…However, with the identification of citrullinecontaining MBP (42), its implication in demyelination in multiple sclerosis (43)(44)(45), and its localization to the intraperiod line of myelin (47,48), it appeared that a division of labor was operative for the plethora of post-translationally modified MBP molecules (4,43). Other functions of MBP charge isomers include stimulation of phospholipase C activity (49,50), actin polymerization in conjunction with Ca 2ϩ -calmodulin (12,51), tubulin stabilization (52), or even potential regulatory roles as transcription factors (8). Presently, MBP is generally believed to be the agent responsible for the autoimmune response that precipitates the active degradation of the myelin sheath in multiple sclerosis (e.g.…”

Myelin basic protein: a multifunctional protein

“…Indeed, it binds to negatively charged lipids so tightly that it must be extracted with acid in order to delipidate it [107] and it can be isolated in a lipid-bound form by extraction with 3-[(3-cholamidopropyl)dimethyl ammonio]-1-propanesulfonate (CHAPS) [100,108]. In addition to negatively charged lipids, MBP interacts with polyanions such as actin filaments [15,17,18,[109][110][111][112] and microtubules in vitro [113,114] and binds to other proteins such as Ca 2+ -calmodulin (CaM) [15,16,[115][116][117][118], tropomyosin [119], and clathrin [120]. The fact that size isoforms of MBP containing the segment encoded by exon II have been found to also localize in the nucleus, suggests that they may bind to polynucleotides [45].…”