FC0105
Myelin basic protein (MBP), 18.5 kDa isoform  -  Fyn SH3

Biological function
MBP (myelin basic protein is a highly positively-charged, developmentally regulated protein family expressed from the gene in the oligodendrocyte lineage, which carries out the adhesion of the cytoplasmic leaflets of compact myelin in central nervous system. During oligodendrocyte development and myelination MBP interacts with Fyn kinase via a central proline-rich region. In addition to SH3 domains, MBP has many binding partners, e.g. calmodulin, cortactin, cytoskeletal proteins.

Structural evidence
S38–S107 peptide (termed as xα2-peptide) of MBP shows poor dispersion in the proton dimension (7.7–8.6 ppm) of the ¹⁵ N-HSQC spectrum. Upon binding to Fyn–SH3 (9.3 kDa), no significant change in secondary structure propensity regarding the overall random coil content of 83.5 % was observed. The chemical shifts of the xα2-peptide were systematically perturbed upon titration with the Fyn–SH3 domain resulting in K_D of low μM range (approx < 10 μM). Relaxation rates indicate that the overall tumbling of the peptide backbone has lowered upon Fyn-SH3 interactions, especially for distinct segments T62–L68 and V83–S99. NOE values reflect more restricted backbone motions on the picosecond-nanosecond timescale.

Biochemical evidence
N- and C-terminal deletion experiments show that binding between MBP and Fyn is non-specific in the absence of the 56 N- terminal amino acids, but is not affected by the absence of 63 C-terminal residues. Cells that were co-transfected with both constitutively-active Fyn and full-length 18.5 kDa MBP exhibited third-degree (tertiary) branching, which dropped upon truncation of 40-50 N-terminal residues. The shorter, 72-107 MBP segment can also bind to Fyn, but the N-terminal flanking region improves affinity.

Structure/Mechanism
xα2-peptide shows minimal secondary structure propensities in the unbound state, which did not change upon Fyn–SH3 binding. MBP transitions to an ensemble of structures that have a more compact global fold, without restricting the secondary structure dynamics. Conversion to PPII conformation was not observed. MBP samples a variety of conformations within the SH3-domain pocket, and that these are modulated by post-translational modifications, such as phosphorylation by MAPKs at T92 and T95. Fuzziness enables phosphorylation to alter the disposition of the central membrane-anchoring α-helix with respect to the membrane, which could modify how the proline-rich region is presented to the cytoplasm for binding. Furthermore, the formation of salt-bridges involving the phosphorylated threonines and basic residues within the proline-rich region can modulate the dynamic properties, flexibility of the ensemble.

Mechanism category
tethering

Significance
Fuzziness enables posttranslational modifications to modulate the conformational properties of the ensemble in the bound form thereby contributing to the conformational switch mechanism and trigger Fyn to the oligodendrocyte membrane.