In humans, the PMP22 gene is located on chromosome 17p11.2 and spans approximately 40kb. The gene contains six exons conserved in both humans and rodents, two of which are 5’ untranslated exons (1a and 1b) and result in two different RNA transcripts with identical coding sequences. The two transcripts differ in their 5' untranslated regions and have their own promoter regulating expression. The remaining exons (2 to 5) include the coding region of the PMP22 gene, and are joined together after post-transcriptional modification (i.e. alternative splicing). The PMP22 protein is characterized by four transmembrane domains, two extracellular loops (ECL1 and ECL2), and one intracellular loop. ECL1 has been suggested to mediate a homophilic interaction between two PMP22 proteins, whereas ECL2 has been shown to mediate a heterophilic interaction between PMP22 protein and Myelin protein zero (MPZ).
Although the PMP22 mechanism of action in myelinating Schwann cells is not fully known, it plays an essential role in the formation and maintenance of compact myelin. When Schwann cells come into contact with a neuronal axon, expression of PMP22 is significantly up-regulated, whereas PMP22 is down-regulated during axonal degeneration or transection. PMP22 has shown association with zonula-occludens 1 and occludin, proteins that are involved in adhesion with other cells and the extracellular matrix, and also support functioning of myelin. Along with cell adhesion function, PMP22 is also up-regulated during Schwann cell proliferation, suggesting a role in cell-cycle regulation. PMP22 is detectable in non-neural tissues, where its expression has been shown to serve as growth-arrest-specific (gas-3) function.
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