<p>Source data for Fig 4A, 4B, and 4E.</p>

Neuronal communication relies on neurotransmitter release from synaptic vesicles. The endocytic protein AP180 is critical for efficient vesicle recycling at presynaptic terminals, and its loss impairs neurotransmission, producing reduced release frequency, enlarged synaptic vesicles, and increased quantal amplitude. Yet how AP180 controls vesicle size and whether vesicle size influences release remains unclear. Here, we show that the C-terminal Assembly domain (AD) of AP180 determines vesicle size and thereby regulates release properties in Caenorhabditis elegans. An AP180 variant lacking the AD (AP180∆AD) increases release frequency, contrasting sharply with the reduced transmission in ap180 null mutants, yet fails to correct the vesicle size or quantal amplitude. These enlarged vesicles evade curvature-dependent inhibition by complexin, a presynaptic regulator of fusion, while remaining dependent on complexin for evoked responses. This selective escape reveals that vesicle size influences release dynamics through curvature-sensing proteins. Replacing the AP180 AD with actin-binding motifs restores normal vesicle size, quantal amplitude, and release frequency, indicating that actin interactions are both necessary and sufficient for AD function. Biochemically, we show that the intrinsically disordered AD forms condensates that enrich actin monomers and nucleate filament assembly, while full-length AP180 couples PIP2-rich membranes to actin filaments. Together, these findings reveal that the AP180 AD regulates synaptic vesicle size through actin binding, establishing vesicle morphology as a key influencer of curvature-dependent release control.神经元通讯依赖于突触囊泡（synaptic vesicles）的神经递质释放。内吞蛋白AP180对于突触前末端的高效囊泡循环至关重要，其缺失会损害神经传递，表现为释放频率降低、突触囊泡增大以及量子振幅（quantal amplitude）升高。然而，AP180如何调控囊泡大小，以及囊泡大小是否会影响神经递质释放，目前仍不明确。本研究表明，AP180的C端组装结构域（Assembly domain，AD）可决定囊泡大小，进而调控秀丽隐杆线虫（Caenorhabditis elegans）的释放特性。一种缺失AD的AP180变体（AP180∆AD）可提升释放频率，这与ap180完全缺失突变体中神经传递降低的表型形成鲜明对比，但该变体无法修复囊泡大小异常以及量子振幅升高的表型。这些增大的囊泡可逃避复合体蛋白（complexin，一类突触前融合调控因子）介导的曲率依赖性抑制，但在诱发神经递质释放的过程中，仍需复合体蛋白的参与。这种选择性逃避现象表明，囊泡大小可通过曲率感知蛋白调控释放动力学。用肌动蛋白结合基序（actin-binding motifs）替换AP180的AD后，可恢复正常的囊泡大小、量子振幅以及释放频率，这表明肌动蛋白相互作用对于AD的功能既是必需的，也是充分的。生化实验显示，固有无序的AD可形成凝聚体，富集肌动蛋白单体并介导肌动蛋白丝组装的成核；而全长AP180可将富含PIP2的细胞膜与肌动蛋白丝连接起来。综上，本研究结果表明，AP180的AD可通过结合肌动蛋白调控突触囊泡大小，从而确立囊泡形态是曲率依赖性释放调控的关键影响因素。