<p>Source data for S5B–S5E, and S5G Fig.</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.神经元信号传递依赖于突触囊泡释放的神经递质。内吞蛋白AP180对于突触前末梢的高效囊泡回收至关重要，其缺失会损伤递质传递，表现为释放频率降低、突触囊泡增大以及量子幅度升高。然而，AP180如何调控囊泡尺寸，以及囊泡尺寸是否影响递质释放，目前仍不明确。本研究显示，在秀丽隐杆线虫（Caenorhabditis elegans）中，AP180的C端组装结构域（Assembly domain, AD）决定囊泡尺寸，进而调控递质释放特性。缺失AD的AP180变体（AP180∆AD）会提升递质释放频率，这与ap180完全敲除突变体中递质传递受损的表型形成鲜明对比，但该变体无法修复囊泡尺寸异常与量子幅度升高的问题。这些增大的囊泡能够逃脱复杂素（complexin，一种突触前融合调节因子）介导的曲率依赖性抑制，但其诱发性释放仍依赖于复杂素。这种选择性逃逸现象表明，囊泡尺寸可通过曲率感知蛋白调控递质释放动力学。将AP180的AD替换为肌动蛋白结合基序后，可恢复正常的囊泡尺寸、量子幅度与释放频率，这表明肌动蛋白相互作用是AD功能所必需且充分的条件。生化实验显示，内在无序的AD可形成凝聚体，富集肌动蛋白单体并促进肌动蛋白丝的成核组装；而全长AP180可将富含磷脂酰肌醇二磷酸（PIP2）的膜与肌动蛋白丝偶联。综上，本研究结果表明，AP180的AD通过结合肌动蛋白调控突触囊泡尺寸，确立了囊泡形态作为曲率依赖性释放调控关键影响因子的地位。