TRIM22 functions as a scaffold protein for autophagy initiation

Tripartite motif (TRIM) family proteins are increasingly recognized as important regulators of autophagy under various physiological and pathological conditions. TRIM22 has been previously shown to mediate autophagosome-lysosome fusion, but its potential role in earlier stages of autophagy remained unexplored. In this study, we investigated the function of TRIM22 in autophagy initiation. Overexpression of TRIM22 increased LC3-II levels and enhanced autophagic flux without affecting mTOR and AMPK activity. We found that TRIM22 interacts with components of both the ULK1 complex and the class III PI3K complex through distinct domains, recruiting them into punctate structures that represent autophagosome formation sites. Domain mapping revealed that the SPRY domain mediates interactions with ATG13 and FIP200, while the N-terminal region interacts with ULK1 and ATG101. The B-box domain of TRIM22 was identified as crucial for its interaction with Beclin-1, a key component of the class III PI3K complex. Deletion of this domain impaired the ability of TRIM22 to assemble the class III PI3K complex and induce autophagic flux. Interestingly, competitive binding assays revealed that Beclin-1 and PLEKHM1 bind to the same region of TRIM22, suggesting a mechanism for coordinating different stages of autophagy. The Alzheimer's disease-associated TRIM22 variant R321K maintained autophagy initiation function in both cell lines and primary neurons. These findings demonstrate that TRIM22 acts as a scaffold protein to promote autophagy initiation, in addition to its previously described role in autophagosome-lysosome fusion. Our study provides new insights into the molecular mechanisms by which TRIM proteins regulate multiple stages of the autophagy process.

三联体基序（Tripartite motif，TRIM）家族蛋白在多种生理及病理状态下作为重要的自噬（autophagy）调控因子受到越来越多的关注。既往研究已证实TRIM22可介导自噬体-溶酶体融合，但其在自噬早期阶段的潜在作用仍未被阐明。本研究旨在探究TRIM22在自噬起始过程中的功能。过表达TRIM22可提升LC3-II水平并增强自噬流，且不影响mTOR与AMPK的活性。研究发现，TRIM22可通过不同结构域分别与ULK1复合物及Ⅲ类PI3K复合物的组分相互结合，并将其招募至代表自噬体形成位点的点状结构中。结构域定位实验显示，SPRY结构域可介导TRIM22与ATG13及FIP200的结合，而N端区域则负责与ULK1及ATG101相互作用。TRIM22的B-box结构域是其与Ⅲ类PI3K复合物核心组分Beclin-1相互结合的关键位点。缺失该结构域会削弱TRIM22组装Ⅲ类PI3K复合物及诱导自噬流的能力。有趣的是，竞争性结合实验结果显示，Beclin-1与PLEKHM1可结合TRIM22的同一区域，这提示了一种协调自噬不同阶段的分子机制。阿尔茨海默病相关的TRIM22突变体R321K在细胞系及原代神经元中均保留了自噬起始功能。上述研究结果表明，TRIM22除了此前报道的介导自噬体-溶酶体融合的功能外，还可作为支架蛋白促进自噬起始过程。本研究为TRIM家族蛋白调控自噬过程多个阶段的分子机制提供了全新的视角。