TRIM28 functions as a negative regulator of aggresome formation

Selective recognition and elimination of misfolded polypeptides are crucial for protein homeostasis. When the ubiquitin-proteasome system is impaired, misfolded polypeptides tend to form small cytosolic aggregates and are transported to the aggresome and eventually eliminated by the autophagy pathway. Despite the importance of this process, the regulation of aggresome formation remains poorly understood. Here, we identify TRIM28/TIF1β/KAP1 (tripartite motif containing 28) as a negative regulator of aggresome formation. Direct interaction between TRIM28 and CTIF (cap binding complex dependent translation initiation factor) leads to inefficient aggresomal targeting of misfolded polypeptides. We also find that either treatment of cells with poly I:C or infection of the cells by influenza A viruses triggers the phosphorylation of TRIM28 at S473 in a way that depends on double-stranded RNA-activated protein kinase. The phosphorylation promotes association of TRIM28 with CTIF, inhibits aggresome formation, and consequently suppresses viral proliferation. Collectively, our data provide compelling evidence that TRIM28 is a negative regulator of aggresome formation. <b>Abbreviations</b>: BAG3: BCL2-associated athanogene 3; CTIF: CBC-dependent translation initiation factor; CED: CTIF-EEF1A1-DCTN1; DCTN1: dynactin subunit 1; EEF1A1: eukaryotic translation elongation factor 1 alpha 1; EIF2AK2: eukaryotic translation initiation factor 2 alpha kinase 2; HDAC6: histone deacetylase 6; IAV: influenza A virus; IP: immunoprecipitation; PLA: proximity ligation assay; polypeptidyl-puro: polypeptidyl-puromycin; qRT-PCR: quantitative reverse-transcription PCR; siRNA: small interfering RNA

错误折叠多肽的选择性识别与清除对于蛋白质稳态（protein homeostasis）至关重要。当泛素-蛋白酶体系统（ubiquitin-proteasome system）功能受损时，错误折叠多肽易形成胞质小聚集体，并被转运至聚集体体（aggresome），最终通过自噬途径（autophagy pathway）被清除。尽管该过程的生物学意义重大，但目前人们对聚集体体形成的调控机制仍知之甚少。本研究鉴定出TRIM28/TIF1β/KAP1（tripartite motif containing 28，三联基序包含蛋白28）为聚集体体形成的负调控因子。TRIM28与CTIF（帽结合复合物依赖的翻译起始因子，cap binding complex dependent translation initiation factor）的直接相互作用，会导致错误折叠多肽的聚集体体靶向效率低下。我们还发现，无论是用聚肌胞苷酸（poly I:C）处理细胞，还是用甲型流感病毒（influenza A viruses）感染细胞，均可通过依赖于双链RNA激活的蛋白激酶的途径，诱导TRIM28的S473位点发生磷酸化。该磷酸化修饰可促进TRIM28与CTIF的结合，抑制聚集体体形成，进而抑制病毒增殖。综上，本研究数据提供了充分证据，表明TRIM28是聚集体体形成的负调控因子。<b>缩写</b>：BAG3：BCL2相关同源物3（BCL2-associated athanogene 3）；CTIF：帽结合复合物依赖的翻译起始因子（cap binding complex dependent translation initiation factor）；CED：CTIF-EEF1A1-DCTN1；DCTN1：动力激活蛋白1亚基1（dynactin subunit 1）；EEF1A1：真核翻译延伸因子1α1（eukaryotic translation elongation factor 1 alpha 1）；EIF2AK2：真核翻译起始因子2α激酶2（eukaryotic translation initiation factor 2 alpha kinase 2）；HDAC6：组蛋白脱乙酰酶6（histone deacetylase 6）；IAV：甲型流感病毒（influenza A virus）；IP：免疫沉淀（immunoprecipitation）；PLA：邻近连接测定法（proximity ligation assay）；polypeptidyl-puro：多肽嘌呤霉素（polypeptidyl-puromycin）；qRT-PCR：定量反转录PCR（quantitative reverse-transcription PCR）；siRNA：小干扰RNA（small interfering RNA）