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SAR1B belongs to the small GTPase superfamily, SAR1 family. It is involved in transport from the endoplasmic reticulum to the Golgi apparatus and is activated by the guanine nucleotide exchange factor PREB. SAR1B is involved in the selection of the protein cargo and the assembly of the COPII coat complex. Defects in SAR1B are the cause of chylomicron retention disease (CMRD). CMRD is an autosomal recessive disorder of severe fat malabsorption associated with failure to thrive in infancy. The conditions are characterized by deficiency of fat-soluble vitamins, low blood cholesterol levels, and a selective absence of chylomicrons from blood. Affected individuals accumulate chylomicron-like particles in membrane-bound compartments of enterocytes, which contain large cytosolic lipid droplets.
