Golgi Transport

The primary focus of our lab is the Golgi complex — a central hub of membrane trafficking in eukaryotic cells. The Golgi complex comprises tightly stacked, flat membrane sacs called cisternae, which are organized into four functional regions: the cis, medial, trans, and trans-Golgi network (TGN). As newly synthesized proteins and lipids (cargos) exit the endoplasmic reticulum (ER), they sequentially traverse these Golgi regions and undergo extensive post-translational modifications, primarily glycosylation. At the TGN, cargos are sorted into membrane carriers for delivery to the endolysosome (via signal-dependent pathways) or to the plasma membrane (via constitutive or default pathways). In addition to its role in the secretory pathway, the Golgi also receives membrane carriers from endosomes as part of the endocytic pathway.

Despite decades of research, the molecular and cellular principles governing Golgi organization and function remain poorly understood. The Golgi remains an enigmatic organelle and a topic of ongoing debate. Our lab is focused on addressing these fundamental questions about the Golgi:

1) Molecular organization of the Golgi.

To investigate Golgi architecture, we developed novel imaging tools. Traditional sub-Golgi localization methods, such as immuno-electron microscopy (immuno-EM), are labor-intensive and require specialized expertise. We introduced GLIM (Golgi Localization by Imaging center of Mass), a super-resolution tool that uses conventional microscopy to achieve nanometer-scale axial (cis-to-trans) resolution, surpassing the precision of immuno-EM (Tie et al., Mol. Biol. Cell, 2016). GLIM enables the systematic and quantitative localization of Golgi proteins, allowing for real-time imaging of intra-Golgi secretory transport — a capability unique to our approach.

We further expanded our toolkit by developing en face and side-view averaging techniques for lateral (rim-to-interior) Golgi localization (Tie et al., eLife, 2018; Tie et al., J. Cell Biol., 2022). These tools revealed a radial organization where Golgi enzymes concentrate at the cisternal interior and trafficking machinery localizes at the cisternal rim. Our suite of quantitative tools now enables systematic, high-resolution analysis of Golgi organization.

2) Intra-Golgi and Peri-Golgi Membrane Trafficking

We apply these tools to study membrane trafficking at and within the Golgi. Using side-view averaging, we directly visualized the intra-Golgi transport of secretory cargos (Tie et al., J. Cell Biol., 2022). We demonstrated that these cargos exit the Golgi at the trans-Golgi, not the TGN, challenging a long-standing model (Tie et al., Mol. Biol. Cell, 2016; Tie et al., J. Cell Biol., 2022). Additionally, we identified the Dopey1-Mon2 complex as a kinesin-1 adaptor that binds phosphatidic acid and PI4P to mediate Golgi-to-periphery transport (Mahajan et al., Nat. Commun., 2019).

3) Regulation of Golgi trafficking

We are also investigating how Golgi trafficking is regulated in response to changes in cellular metabolic states. We discovered that endosome-to-Golgi trafficking is modulated by amino acid availability — promoted by sufficiency and suppressed by starvation. We found that SLC38A9 and v-ATPase sense amino acids and activate Arl5 through Ragulator, linking nutrient sensing to membrane trafficking (Shi et al., Nat. Commun., 2018). Arl5 likely cooperates with the GARP tethering complex to mediate endosome-to-Golgi trafficking. Given the critical roles of the endosome-to-Golgi trafficking pathway in development, toxin entry, cellular homeostasis, and disease, our findings suggest a potential role for nutrient signaling in modulating disease progression.