And Yet it is Modified-Holding a Candle to the Dark Matter of White Matter.

The multilamellar membrane myelin sheath of the CNS, that enwraps axons to facilitate saltatory conduction in higher vertebrates, is held together by myelin basic protein (MBP). Yet this generalization masks how enigmatic MBP is, much like cosmological "dark matter." First, the casual use of the singular form for "protein" distracts that there are multiple, developmentally regulated "classic" splice isoforms ranging from 14 to 21.5 kDa, each with extensive PTMs. Second, the static image of MBP adhering two cytoplasmic leaflets of the oligodendrocyte membrane together in close apposition, suggests it to be inaccessible to modifying enzymes. And yet it is modified (to paraphrase Galileo's phrase on the earth's motion). In this issue of Proteomics, Sarg et al. apply an integrated CE-MS approach to investigate the PTMs of 18.5 kDa MBP from mouse brains of different ages. They identify new sites and types of modification, as well as confirming previously known PTMs. Innovative tools for unraveling the intricacies of the myelin basic proteome and how it organizes CNS myelin (much like basic histones organize chromatin), will help us understand white matter development and plasticity in health, during ageing, and in demyelinating diseases such as multiple sclerosis.