The Classic Basic Protein of Myelin – Conserved Structural Motifs and the Dynamic Molecular Barcode Involved in Membrane Adhesion and Protein-Protein Interactions

Abstract

The myelin basic protein (MBP) family comprises a variety of developmentally-regulated members arising from different transcription start sites, differential splicing, and post-translational modifications. The "classic" isoforms of MBP include the 18.5 kDa form, which predominates in adult human myelin and facilitates compaction of the mature myelin sheath in the central nervous system, thereby maintaining its structural integrity. In addition to membrane-association, the 18.5 kDa and all other classic isoforms are able to interact with a multitude of proteins, including Ca(2+)-calmodulin, actin, tubulin, and SH3-domain containing proteins, and thus may be signalling linkers during myelin development and remodelling. All proteins in this family are intrinsically disordered, creating a large effective surface to facilitate multiple protein associations, and are post-translationally modified to various degrees by methylation, phosphorylation, and deimination. We have used spectroscopic (fluorescence, CD, EPR, and NMR) approaches to study MBP's conformational adaptability. A highly-conserved central domain presents an amphipathic alpha-helix in association with a phospholipid membrane, and contains a threonyl residue that is phosphorylated by MAP-kinases. In multiple sclerosis, this segment represents a primary immunodominant epitope. This helical structure is adjacent to a proline-rich region that presents a classic SH3-ligand, comprises a second MAP-kinase phosphorylation site, and forms a polyproline type II helix. This domain of the protein is thus essential to proper positioning of a protein-interaction motif, with the local conformation and accessibility being modulated by MAP-kinases. In addition, the C-terminus of 18.5 kDa MBP has been identified by NMR spectroscopy as a Ca(2+)-calmodulin-binding site, and is of note for having a high density of post-translational modifications (protein kinase C phosphorylation, and deimination). For the most part, any classic protein isoform functions as an entropic spring that interacts in its entirety with membranes and cytoskeletal proteins, but the central and C-terminal motifs may represent molecular switches.

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 CNS myelin, although mostly composed of lipids, possesses many fundamental proteins which are essential for proper structural and functional integrity of the sheath (reviewed by [7]). Among these essential proteins are the myelin basic proteins (MBPs) [8][9][10][11][12][13], a family composed of developmentally regulated members arising from different transcription start sites of the Golli (Gene of Oligodendrocyte Lineage) complex, with further alternatively-spliced isoforms and combinatorial post-translational modifications [14]. 'Classic' MBP isoforms arise from transcription start site 3 and range in size from 14 kDa to the full-length 21.5-kDa transcript.…”

“…'Classic' MBP isoforms arise from transcription start site 3 and range in size from 14 kDa to the full-length 21.5-kDa transcript. They also undergo combinatorial post-translational modifications including phosphorylation and deimination of arginine, resulting in a number of charge components [11]. They adapt structurally and bind to a variety of different polyanionic proteins such as actin and tubulin, and also have been shown to have other important interactions with calmodulin and SH3-domain proteins such as Fyn kinase, the actin-polymerizing protein cortactin, and PSD-95 (presynaptic density protein 95) [8,[10][11][12][15][16][17][18].…”

The 21.5-kDa isoform of myelin basic protein has a non-traditional PY-nuclear-localization signal

“…Although the 21.5-kDa MBP isoform is intrinsically disordered like 18.5-kDa MBP, far less is known about its structural dynamics and possible self-association, and it is worthwhile to explore the effects of these mutations by NMR spectroscopy [14,15,44]. It has also been suggested that the phosphorylation status of several potential protein kinase C (PKC) phosphorylation sites within the N-terminal region of MBP may modulate their subcellular localization in OLGs [20,22,[31][32][33]45], another phenomenon worthy of further exploration.…”

“…Basic residues are colored blue and acidic residues are colored red. The amino acid numbering omits the Nterminal methionyl residue which is cleaved post-translationally, and follows our standard convention [15][16][17]. (B) ClustalW2 alignment of the conserved regions encoded by exon-II of 21.5-kDa MBP in various species.…”

“…Its main purpose is adhesion of the cytoplasmic leaflets of OLG membranes [8] to form a "molecular sieve" [10,11]. However, this protein is multifunctional and interacts with numerous other proteins including actin, tubulin, Ca 2+ -activated calmodulin, and SH3-domain proteins (reviewed in [12][13][14][15][16][17][18][19]). …”