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Coat protein complex II (COPII) plays an integral role in the packaging of secretory cargoes within membrane-enclosed transport carriers that leave the endoplasmic reticulum (ER) from discrete subdomains. Lipid bilayer remodeling necessary for this process is driven initially by membrane penetration mediated by the Sar1 GTPase and further stabilized by assembly of a multilayered complex of several COPII proteins. However, the relative contributions of these distinct factors to transport carrier formation and protein trafficking remain unclear. Here, we demonstrate that anterograde cargo transport from the ER continues in the absence of Sar1, although the efficiency of this process is dramatically reduced. Specifically, secretory cargoes are retained nearly five times longer at ER subdomains when Sar1 is depleted, but they ultimately remain capable of being translocated to the perinuclear region of cells. Taken together, our findings highlight alternative mechanisms by which COPII promotes transport carrier biogenesis. [Display omitted] • Loss of Sar1 fails to inhibit the export of multiple secretory cargoes from the ER • The inner COPII coat complex is indispensable for anterograde protein transport • COPII coat proteins undergo phase transition in the absence of Sar1 In this study, Kasberg et al. demonstrate that the Sar1 GTPase only regulates the kinetics of anterograde protein trafficking from the ER but is not required for this pathway. These findings highlight the existence of a previously uncharacterized, alternative mode of secretory protein transport between the ER and the Golgi. [ABSTRACT FROM AUTHOR]