Structures of Two Melanoma-Associated Antigens Suggest Allosteric Regulation of Effector Binding

The MAGE (melanoma associated antigen) protein family are tumour-associated proteins normally present only in reproductive tissues such as germ cells of the testis. The human genome encodes over 60 MAGE genes of which one class (containing MAGE-A3 and MAGE-A4) are exclusively expressed in tumours, making them an attractive target for the development of targeted and immunotherapeutic cancer treatments. Some MAGE proteins are thought to play an active role in driving cancer, modulating the activity of E3 ubiquitin ligases on targets related to apoptosis. Here we determined the crystal structures of MAGE-A3 and MAGE-A4. Both proteins crystallized with a terminal peptide bound in a deep cleft between two tandem-arranged winged helix domains. MAGE-A3 (but not MAGE-A4), is predominantly dimeric in solution. Comparison of MAGE-A3 and MAGE-A3 with a structure of an effector-bound MAGE-G1 suggests that a major conformational rearrangement is required for binding, and that this conformational plasticity may be targeted by allosteric binders.MAGE（黑色素瘤相关抗原，melanoma associated antigen）蛋白家族为一类肿瘤相关蛋白，正常生理状态下仅表达于睾丸生殖细胞等生殖组织中。人类基因组编码超过60种MAGE基因，其中一类（包含MAGE-A3与MAGE-A4）仅在肿瘤组织中特异性表达，使其成为靶向性免疫抗肿瘤疗法开发的极具吸引力的候选靶点。部分MAGE蛋白被认为在肿瘤发生过程中发挥主动调控作用，可调节E3泛素连接酶对细胞凋亡相关靶点的活性。本研究解析了MAGE-A3与MAGE-A4的晶体结构：两种蛋白均结合有一段末端肽段，该肽段坐落于两个串联排布的翼螺旋结构域之间的深裂隙内。MAGE-A3（而非MAGE-A4）在溶液中主要以二聚体形式存在。通过将MAGE-A3、MAGE-A4与效应蛋白结合的MAGE-G1结构进行比对分析，结果提示配体结合需要发生显著的构象重排，且这种构象可塑性可作为变构结合剂的靶向作用位点。