The myelin proteins of the shark brain are similar to the myelin proteins of the mammalian peripheral nervous system

The two major structural proteins in the shark CNS are similar to the structural proteins, Po and myelin basic protein (MBP), found in the mammalian peripheral nervous system (PNS). Shark Po is 46% similar to its mammalian counterpart. The extracellular domain of shark Po also appears to be organized as an immunoglobulin-like domain that mediates homotypic interactions. The intracellular domain of shark Po also is very basic and may play a role in myelin condensation analogous to that of MBP. Shark MBP is 44% similar to mammalian MBP. Both MBPs show conserved interspersed regions and are present in multiple forms that arise by alternative splicing of a single transcript. These structural analyses indicate that the complexities seen in mammalian myelin arose early during vertebrate evolution.