Anterograde transport in eukaryotic cells begins at the endoplasmic reticulum (ER), where cargo proteins that are synthesized at the ER, become packaged into small vesicles for transport to other compartments in the cell. The coat protein complex II (COPII) is a set of proteins responsible for selecting cargo destined to leave the ER, and forming these vesicles from the ER membrane. In mammalian cells, COPII proteins have diversified, as gene duplication events have created multiple paralogs for most of these COPII components. The COPII subunit Sec24 is thought to be the major cargo binding subunit, but the activity and specificity for each of its 4 paralogs in mammalian cells is unknown.

Here I report on the discovery that a mutation in Sec24B leads to a major neural tube defect in mice. Neural tube defects are often the result of deficiencies in a tissue organization pathway called planar cell polarity. Using an in vitro reconstitution of COPII vesicle formation, I was able to identify a unique cargo protein that is packaged specifically by Sec24B, the planar cell polarity protein Vangl2. The study of Vangl2 has often centered on mice with specific point mutations called the looptail mutant mice. These mice have phenotypes commonly found associated with defects in planar cell polarity, but the cellular reason for these defects was unclear. I demonstrate that the looptail mutant Vangl2 protein is restricted to the ER and unable to be packaged into COPII vesicles. I further characterized the Vangl2 protein and its looptail mutant forms, identifying many more mutations that result in transport defects. I probed Vangl2 oligomerization, topology, folding status, and binding partners. These results establish a special role for Sec24B in mammalian development, specifically in the transport of Vangl2, and further illustrate the specificity of cargo protein recruitment and transport competence.