Objective The Golgi apparatus serves as a central hub in the eukaryotic secretory pathway, responsible for the processing, sorting, and trafficking of proteins and lipids. In mammalian cells, the Golgi typically forms a perinuclear ribbon-like structure composed of laterally connected cisternae stacks.The maintenance of Golgi ribbon structure depends on the balance of membrane flux across multiple intracellular trafficking pathways, yet the specific contributions of distinct trafficking branches to Golgi macroscopic morphology remain elusive. In mammalian cells, the Golgi ribbon is typically organized as a perinuclear, laterally connected structure composed of stacked cisternae, and its integrity is highly dynamic and sensitive to perturbations in membrane trafficking. This study aims to elucidate the role of coat protein complex I (COPI)-mediated retrograde transport in maintaining the Golgi ribbon and to dissect the functional relationship between the transmembrane cargo receptors LEPROT/LEPROTL1 (LEPROTs) and the COPI adaptor GOLPH3.Methods Using siRNA interference and gene-deficient cell lines, we selectively perturbed COPI- or adaptor protein complex 1 (AP-1)-mediated trafficking pathways in HeLa cells. To quantitatively evaluate Golgi morphology, we employed a “Golgi Angle”-based measurement to assess its circumferential distribution around the nucleus. The spatial distribution of the Golgi ribbon was quantitatively analyzed using confocal microscopy, while Golgi ultrastructure and vesicle density were examined via transmission electron microscopy. Additionally, the subcellular distribution of COPI components was assessed by immunofluorescence co-localization.Results Selective inhibition of COPI retrograde transport significantly induced the circumferential extension of the Golgi ribbon around the nucleus, whereas blocking AP-1-mediated anterograde transport resulted in Golgi compaction, indicating opposing roles. These results suggest that different trafficking branches downstream of ARF1 exert distinct and even antagonistic effects on Golgi morphology. LEPROTs-deficient cells exhibited a Golgi extension phenotype highly consistent with COPI impairment. Furthermore, knockdown of GOLPH3 in a LEPROTs double-knockout background produced a significant additive effect on Golgi extension, suggesting that LEPROTs and GOLPH3 play non-redundant roles in regulating COPI-related trafficking processes. Mechanistically, loss of either LEPROTs or GOLPH3 led to the aberrant accumulation of COPI components at endoplasmic reticulum exit sites, accompanied by a reduction in COPI-like vesicles around the Golgi. This redistribution indicates a defect in COPI recycling between the ER-Golgi interface and the Golgi apparatus. Ultrastructural analysis revealed that Golgi cisternae in defective cells became shorter and thicker while maintaining a stable number of stacks. In parallel, the density of Golgi-associated vesicles was markedly decreased, further supporting an impairment in COPI vesicle formation or budding processes.Conclusion This study demonstrates that active COPI retrograde transport is a critical factor in restricting the over-connection of the Golgi ribbon and maintaining its compactness. Rather than causing fragmentation, partial disruption of COPI function leads to a distinct morphological outcome characterized by Golgi ribbon extension at the light microscopy level and cisternal remodeling at the ultrastructural level. LEPROTs and GOLPH3 cooperatively promote the recycling and vesiculation of COPI components, thereby imposing a structural constraint on the Golgi periphery. Our findings support a model in which multiple adaptor proteins act in parallel to sustain efficient COPI cycling, thereby maintaining Golgi structural homeostasis. These findings provide new cell biological evidence for the membrane trafficking basis of Golgi morphological homeostasis.