MYD88是Toll样和白介素-1受体的关键信号适配蛋白。MYD88的Toll/白介素-1受体(TIR)结构域内的体细胞L265P突变在90%的瓦尔登斯特伦大球蛋白血症和一部分弥漫性大B细胞淋巴瘤中发现。MYD88-L265P强烈促进NF-κB通路激活、JAK-STAT信号以及淋巴瘤细胞生存。先前的研究已经确认了TIR结构域中的其他残基在NF-κB激活中起到了至关重要的作用，包括丝氨酸257 (S257)，表明其在调控MYD88激活中可能具有重要的生理作用。在这里，我们证明了B细胞淋巴瘤细胞中的MYD88 S257被磷酸化，而且这种磷酸化对于TLR诱导的NF-κB激活是必需的。此外，我们证明了磷酸模拟的MYD88-S257D突变体能够促进MYD88聚集、IRAK1磷酸化、NF-κB激活和细胞生长，与致癌性的L265P突变体相差无几。最后，我们展示了MYD88-S257D的表达可以在对致癌性MYD88信号依赖的淋巴瘤细胞中静默内源性MYD88-L265P表达的情况下恢复细胞生长。我们的数据表明，L265P突变通过模拟MY88在S257位点的磷酸化效应，促进TIR结构域同源二聚化和NF-κB激活，从而揭示了MYD88-L265P在B细胞恶性肿瘤中致癌活性的分子机制的新见解。© 2023. Springer Nature Limited.

MYD88 is the key signaling adaptor-protein for Toll-like and interleukin-1 receptors. A somatic L265P mutation within the Toll/interleukin-1 receptor (TIR) domain of MYD88 is found in 90% of Waldenström macroglobulinemia cases and in a significant subset of diffuse large B-cell lymphomas. MYD88-L265P strongly promotes NF-κB pathway activation, JAK-STAT signaling and lymphoma cell survival. Previous studies have identified other residues of the TIR-domain crucially involved in NF-κB activation, including serine 257 (S257), indicating a potentially important physiological role in the regulation of MYD88 activation. Here, we demonstrate that MYD88 S257 is phosphorylated in B-cell lymphoma cells and that this phosphorylation is required for optimal TLR-induced NF-κB activation. Furthermore, we demonstrate that a phosphomimetic MYD88-S257D mutant promotes MYD88 aggregation, IRAK1 phosphorylation, NF-κB activation and cell growth to a similar extent as the oncogenic L265P mutant. Lastly, we show that expression of MYD88-S257D can rescue cell growth upon silencing of endogenous MYD88-L265P expression in lymphoma cells addicted to oncogenic MYD88 signaling. Our data suggest that the L265P mutation promotes TIR domain homodimerization and NF-κB activation by copying the effect of MY88 phosphorylation at S257, thus providing novel insights into the molecular mechanism underlying the oncogenic activity of MYD88-L265P in B-cell malignancies.© 2023. Springer Nature Limited.