Defining the Roles of Mammalian COPII Components SEC24C and SEC24D.

Abstract

Newly synthesized proteins are transported from the endoplasmic reticulum (ER) to the Golgi via COPII coated vesicles. SEC24 is the COPII component primarily responsible for the recruitment of transmembrane cargoes or cargo adaptors into newly forming COPII vesicles on the ER membrane. Mammalian genomes encode four Sec24 paralogs (Sec24a-d), though little is known about their comparative functions. Based on protein sequence, SEC24A/B are more closely related to one another than they are to SEC24C/D. Mice deficient for SEC24A exhibit a mild hypocholesterolemic phenotype, and Sec24b null embryos die from a specific neural tube closure defect near the end of embryonic development. Through the generation and characterization of a mouse model deficient in SEC24C, we demonstrate that SEC24C is required during early embryonic development, with SEC24C-deficient embryos dying around embryonic day 7.5. We found SEC24C to be dispensable in a number of tissues, likely as a result of compensation by other Sec24 paralogs. A mouse model of SEC24D-deficiency revealed that the requirement for SEC24D begins prior to the blastocyst stage. Mice heterozygous for null alleles of Sec24c or Sec24d, as well as compound heterozygotes, exhibit no phenotypic abnormalities. To determine the extent of functional overlap between SEC24C and SEC24D, we generated a Sec24cc-d allele, in which the SEC24C coding sequence has been largely replaced with SEC24D. We found that SEC24D, when its expression is driven by Sec24c regulatory elements, can rescue Sec24c null mice from embryonic lethality. However, Sec24cc-d /c-d mice die shortly after birth, suggesting that the overlap in function between SEC24C and SEC24D is incomplete, consistent with the wide range of phenotypes observed in mouse models of SEC24 deficiency. Taken together, these results indicate that the four Sec24 paralogs have developed unique functions over the course of vertebrate evolution, but have also maintained some overlap in function.