The secretory pathway of eukaryotic cells packages cargo proteins into COPII-coated vesicles for transport from the endoplasmic reticulum (ER) to the Golgi. We now report that complete genetic deficiency for the COPII component SEC24A is compatible with normal survival and development in the mouse, despite the fundamental role of SEC24 in COPII vesicle formation and cargo recruitment. However, these animals exhibit markedly reduced plasma cholesterol, with mutations in Apoe and Ldlr epistatic to Sec24a, suggesting a receptor-mediated lipoprotein clearance mechanism. Consistent with these data, hepatic LDLR levels are up-regulated in SEC24A-deficient cells as a consequence of specific dependence of PCSK9, a negative regulator of LDLR, on SEC24A for efficient exit from the ER. Our findings also identify partial overlap in cargo selectivity between SEC24A and SEC24B, suggesting a previously unappreciated heterogeneity in the recruitment of secretory proteins to the COPII vesicles that extends to soluble as well as trans-membrane cargoes.
The endoplasmic reticulum (ER) is a structure that performs a variety of functions within eukaryotic cells. It can be divided into two regions: the surface of the rough ER is coated with ribosomes that manufacture various proteins, while the smooth ER is involved in activities such as lipid synthesis and carbohydrate metabolism. Proteins synthesized by the ribosomes attached to the rough ER are generally transferred to another structure within the cell, the Golgi apparatus, where they undergo further processing and packaging before being secreted or transported to another location within the cell.
Proteins are shuttled from the ER to the Golgi apparatus by vesicles covered with coat protein complex II (COPII). This complex is composed of an inner and outer coat, each of which is assembled primarily with two different SEC proteins: the SEC23/SEC24 protein heterodimer forms the inner coat of the COPII vesicle, and plays a key role in recruiting the appropriate protein cargos to the transport vesicle, while the SEC13/SEC31 protein heterotetramer forms the outer coat and is generally responsible for regulating vesicle size and rigidity.
Previous work found that mammals, including humans and mice, harbor multiple copies of several SEC protein genes, including two copies of SEC23 and four copies of SEC24. Both copies of SEC23 are derived from the same ancestral gene, and all four copies of SEC24 are derived from a different ancestral gene, and the availability of these copies potentially expands the range of properties that the vesicles can have. Insight into the roles of each SEC protein has come from work with SEC mutants. For example, a mutation in SEC23A was found to cause skeletal abnormalities in humans.
Here, Chen et al. report the results of experiments which showed that mice with an inactive Sec24a gene could develop normally. However, these mice experienced a 45% reduction in their plasma cholesterol levels because they were not able to recruit and transport a secretory protein called PCSK9, which is a critical regulator of blood cholesterol levels.
The work of Chen et al. reveals a previously unappreciated complexity in the recruitment of secretory proteins to the COPII vesicle and suggests that the various combinations of SEC proteins influence the proteins selected for transport to the Golgi apparatus. The work also identifies Sec24a as a potential therapeutic target for the reduction of plasma cholesterol, a finding that could be of interest to researchers working on heart disease and other conditions exacerbated by high cholesterol.