Myelin basic proteins charge isomers interact differently with the peptidyl arginine deiminase-2

The deamination of arginine and its conversion to citrulline is a modification observed in positively charged proteins such as histones or myelin basic protein (MBP). This reaction is catalyzed by peptidyl arginine deiminase (PAD), whose abnormal activation is associated with autoimmune diseases like rheumatoid arthritis and multiple sclerosis. However, the mechanisms that trigger PAD activation and the pathophysiological processes involved in hypercitrullination remain unknown. In this study, we investigated the interaction between PAD and various charged isomers of MBP, each differing in the degree of post-translational modification. Immunoprecipitation experiments were conducted to examine the binding between PAD and the different charge isomers of MBP. Our findings revealed that the phosphorylated forms of MBP (C3 and C4) exhibited a higher affinity for PAD compared to the unmodified (C1) and fully citrullinated forms (C8). Additionally, we observed that only in the presence of the unmodified C1 isomer did PAD undergo autocitrullination, which was inhibited by the endogenous guanidine-containing component, creatine. In the presence of other isomers, PAD did not undergo autocitrullination. Furthermore, we found that the unmodified isomer of MBP-C1 contains methylated arginines, which were not affected by the pre-treatment with PAD. Based on our findings, we propose that the increased phosphorylation of central threonines in the original MBP may trigger PAD activation, leading to increased citrullination of the protein and subsequent disorganization of the myelin sheath. These insights contribute to a better understanding of the underlying mechanisms in autoimmune diseases associated with hypercitrullination, potentially opening new avenues for therapeutic interventions.