Cooperative definition of ER exit sites by Tango1, Sec16 and Sec12

Endoplasmic reticulum (ER) exit sites (ERES) are dynamic ER subdomains where secretory cargos are concentrated and exported towards the Golgi apparatus. The conserved proteins Sec16 and Sec12 have long been implicated in ERES organization, while animal-specific Tango1 has more recently emerged as a possible additional determinant. Their respective roles in ERES morphogenesis, however, remain unclear. Using Drosophila, we dissect the mutual requirements and specific functions of Sec16, Sec12 and Tango1 in defining ERES. We show that Sec16 and Tango1 are interdependent for proper ERES assembly, with co-overexpression producing exaggerated ERES structures. We further identify CG9175 as the Drosophila ortholog of Sec12 and demonstrate its upstream requirement for coordinated concentration of Tango1 and Sec16. Tango1 is in turn important for ERES-Golgi coupling. Finally, both Tango1 and Sec16are required for concentration of COPII and COPI components, albeit with different relative impacts. Together, our findings support the evolutionary incorporation of Tango1 into a cooperative network in which Sec12, Sec16 and Tango1 make distinct contributions to the definition of animal ERES.