1.
Myelin basic protein (MBP) is a family of developmentally-regulated proteins that arise from different transcription start sites of the Golli (genes of the oligodendrocyte lineage) gene complex, and by alternate splicing to generate different isoforms. The best-studied member of this family is the 18.5 kDa "classic" MBP, that is preponderant in adult human and bovine myelin. Further diversity of this MBP isoform is achieved by a plethora of post-translational modifications: Nterminal acylation, methylation, deamidation, phosphorylation, ADP-ribosylation, and deimination (the irreversible enzymatic conversion of arginine to citrulline). These changes result in the cumulative reduction of MBP's net positive charge, and a protein preparation from myelin can be separated by ion exchange chromatography into a series of charge components or isomers denoted C1 to C8. The least-modified form, C1, has a net charge of +19 at neutral pH, and predominates in normal adult myelin. The most-modified form, C8, has a net charge of +13 at neutral pH and is primarily deiminated. Intermediate forms are deamidated and phosphorylated. MBP is a candidate autoantigen in multiple sclerosis (MS), in which the proportion of the deiminated isoform C8 is increased. Interestingly, the proportion of C8 is increased also in developing myelin in children less than 5 years of age. Structurally, MBP is a highly-flexible and extended molecule of the emerging "intrinsically-unstructured" or "conformationally adaptable" class that is now known to comprise several hundred proteins, including many involved in signalling. In other words, the protein is designed to interact with numerous other molecules via a "fly-casting" mechanism, that facilitates rapid association with no loss of specificity. The main function of the classic 18.5 kDa MBP is to hold the cytoplasmic leaflets of the oligodendrocyte membrane processes in close apposition, thus maintaining the structural integrity of the myelin sheath. Charge reduction by post-translational modification attenuates this interaction. In particular, we have shown by electron paramagnetic resonance (EPR) spectroscopy that a central immunodominant epitope of MBP forms a surface-associated amphipathic alpha-helix. Upon deimination of the protein to the C8 form, this epitope becomes more exposed, and the carboxyterminus becomes expelled from the membrane.
The classic 18.5 kDa MBP interacts with calmodulin, actin, tubulin, clathrin, and phospholipase C, and we have shown that several of these interactions are modulated by charge reduction due to protein modification. We have also recently found that phosphorylated MBP is localised in detergent-resistant membrane microdomains in adult myelin, whereas methylated and deiminated MBP are found in bulk myelin. We are currently investigating the pattern of MBP modification in two spontaneously demyelinating transgenic mouse lines that are models for MS. In one of these, the enzyme peptidylarginine deiminase II is overexpressed under the control of the MBP promoter. The characterisation of this mouse line will be presented.
Our studies thus support the conjecture that MBP functions also in signal transduction mechanisms involved in cytoskeletal organisation. We postulate further that there is an equilibrium amongst the classic charge isomers in adult myelin, with deimination representing a means for protein turnover, and methylation and phosphorylation representing a means for modulating MBP's interactions with actin, tubulin, and other molecules. In MS, this equilibrium is shifted towards deimination, resulting not only in instability of myelin and its physical deterioration, but in disruption of signalling pathways.

2.
ペプチジルアルギニンデイミナーゼ(PAD)は、ペプチド中のアルギニンをシトルリンに変換する翻訳後修飾酵素である。シトルリンを含む蛋白質をシトルリン化蛋白質と総称する。最近、シトルリン化蛋白質が関節リウマチや多発性硬化症の原因であると考えられるようになった。我々は、これら疾患以外にもアルツハイマー病や乾癬、アトピー性皮膚炎にもシトルリン化蛋白質が関与することを明らかにしている。最近の知見について報告したい。