Modulation of the sizes of COPII/KLHL12-coated vesicles
COPII proteins (SAR1, SEC23/24, and SEC13/31) carve the endoplasmic reticulum (ER) membrane into transport vesicles that are typically about 60-80 nm in diameter and package cargos into the budding vesicles. Procollagen forms about a 300-nm-long rigid fibril in the ER and requires COPII proteins for export from the ER. Thus, COPII proteins should generate larger carriers to accommodate procollagen. It has been recently shown that monoubiquitylation of SEC31 by CUL3-KLHL12 is essential for ER export of procollagen. Overexpression of KLHL12 drives the assembly of COPII/KLHL12-coated megavesicles and expedites secretion of procollagen. However, how COPII proteins generate such vesicles remains unanswered. Cranio-lenticulo-sutural-dysplasia (CLSD), an autosomal recessive syndrome, is caused in part by defective deposition of collagen. Homozygous F382L SEC23A mutation has been identified in the original consanguineous family by positional cloning.
Recently, we described a new CLSD case with heterozygous M702V SEC23A mutation that was inherited from clinically normal father. Expression of M702V SEC23A partially attenuates ER export of procollagen. Purified recombinant M702V SEC23A, however, interacts normally with other COPII components and efficiently packages the smaller cargo molecules that we have tested. Interestingly, M702V SEC23A accelerates SAR1B GTP hydrolysis when SEC31 is present. Because accelerated SAR1 GTP hydrolysis leads to premature release of COPII proteins from the membrane, premature release of COPII proteins from the membrane may lead to formation of smaller COPII vesicles and exclusion of procollagen from nascent vesicles. Remarkably, when we overexpressed KLHL12, the sizes of KLHL12-coated vesicles became smaller in the M702V patient’s fibroblasts than in control fibroblasts. We, therefore, hypothesize that the rate of SAR1 GTP hydrolysis controls the sizes of COPII/KLHL12-coated megavesicles.