Cellular mRNA recruits the ribosome via eIF3-PABP bridge to initiate internal translation

IRES-mediated translation of key cell fate regulating genes has been implicated in tumorigenesis. Concerted action of canonical eukaryotic initiation factors and IRES transacting factors (ITAFs) was shown to regulate cellular IRES mediated translation; however, the precise molecular mechanism of ribosome recruitment to cellular IRESes remains unclear. Here we show that the X-linked inhibitor of apoptosis (XIAP) IRES operates in an evolutionary conserved viral like mode and the structural integrity, particularly in the vicinity of AUG, is critical for ribosome recruitment. The binding of eIF3 together with PABP potentiates ribosome recruitment to the IRES. Our data support the model in which eIF3 binds directly to the XIAP IRES RNA in a structure-dependent manner and acts as a scaffold for IRES RNA, PABP and the 40S ribosome.

依赖内部核糖体进入位点（Internal Ribosome Entry Site, IRES）的关键细胞命运调控基因翻译，已被证实与肿瘤发生密切相关。已有研究表明，经典真核起始因子与IRES反式作用因子（IRES transacting factors, ITAFs）的协同作用，可调控细胞IRES介导的翻译过程；但核糖体招募至细胞IRES的精确分子机制仍不明晰。本研究发现，X连锁凋亡抑制蛋白（X-linked inhibitor of apoptosis, XIAP）的IRES以进化保守的类病毒模式行使功能，且其结构完整性——尤其是在AUG起始密码子附近区域——对核糖体招募至关重要。真核起始因子3（eukaryotic initiation factor 3, eIF3）与多聚腺苷酸结合蛋白（poly(A)-binding protein, PABP）的结合，可增强核糖体对该IRES的招募效率。我们的数据支持如下模型：eIF3以结构依赖的方式直接结合XIAP IRES RNA，并作为IRES RNA、PABP与40S核糖体亚基的支架蛋白发挥作用。