mRNA cap guanine-N7 methyltransferase (RNMT) catalyzes the S-adenosyl-dependent methylation of the 5’ cap on mRNA at the N-7 position of guanosine. RNA guanine-N7 methyltransferase activating subunit (RAM) allosterically binds to RNMT, which enhances its methyltransferase activity. RAM is phosphorylated at Ser36; however, how this post-translational modification impacts its interaction with RNMT is still unclear. Ser36 of RAM is positioned within a positively charged binding pocket of RNMT, indicating that phosphorylation would improve the binding affinity between these two proteins. Using protein semi-synthesis, we discovered that the first 45 amino acids of RAM is sufficient for full binding to RNMT, and that phosphorylation of Ser36 does increase the binding affinity around six-fold. / 2026-02-28T00:00:00Z