Ludwig Institute for Cancer Research Brussels, Brussels, Belgium.
Université catholique de Louvain and de Duve Institute, Brussels, Belgium.
Nat Commun. 2023 Apr 5;14(1):1881. doi: 10.1038/s41467-023-37277-3.
Calreticulin (CALR) frameshift mutations represent the second cause of myeloproliferative neoplasms (MPN). In healthy cells, CALR transiently and non-specifically interacts with immature N-glycosylated proteins through its N-terminal domain. Conversely, CALR frameshift mutants turn into rogue cytokines by stably and specifically interacting with the Thrombopoietin Receptor (TpoR), inducing its constitutive activation. Here, we identify the basis of the acquired specificity of CALR mutants for TpoR and define the mechanisms by which complex formation triggers TpoR dimerization and activation. Our work reveals that CALR mutant C-terminus unmasks CALR N-terminal domain, rendering it more accessible to bind immature N-glycans on TpoR. We further find that the basic mutant C-terminus is partially α-helical and define how its α-helical segment concomitantly binds acidic patches of TpoR extracellular domain and induces dimerization of both CALR mutant and TpoR. Finally, we propose a model of the tetrameric TpoR-CALR mutant complex and identify potentially targetable sites.
钙网织蛋白（CALR）移码突变是骨髓增殖性肿瘤（MPN）的第二个致病原因。在健康细胞中，CALR 通过其 N 端结构域短暂且非特异性地与未成熟的 N 糖基化蛋白相互作用。相反，CALR 移码突变体通过与血小板生成素受体（TpoR）稳定且特异性地相互作用，变成流氓细胞因子，从而诱导其组成性激活。在这里，我们确定了 CALR 突变体获得的与 TpoR 特异性结合的基础，并定义了复合物形成触发 TpoR 二聚化和激活的机制。我们的工作揭示了 CALR 突变体 C 端使 CALR N 端结构域暴露，使其更容易与 TpoR 上未成熟的 N 聚糖结合。我们进一步发现，碱性突变体 C 端部分呈α-螺旋结构，并定义了其α-螺旋片段如何同时结合 TpoR 细胞外结构域的酸性斑块，并诱导 CALR 突变体和 TpoR 的二聚化。最后，我们提出了一个四聚体 TpoR-CALR 突变体复合物的模型，并确定了潜在的靶标。
