In eukaryotic cells, newly synthesized secretory proteins require COPII (coat protein complex II) to exit the endoplasmic reticulum (ER). COPII contains five core components: SAR1, SEC23, SEC24, SEC13, and SEC31. SEC23 is a GTPase-activating protein that activates the SAR1 GTPase and also plays a role in cargo recognition. Missense mutations in the human COPII paralogues SEC23A and SEC23B result in craniolenticulosutural dysplasia and congenital dyserythropoietic anemia type II, respectively. We now report that mice completely deficient for SEC23B are born with no apparent anemia phenotype, but die shortly after birth, with degeneration of professional secretory tissues. In SEC23B-deficient embryonic pancreas, defects occur in exocrine and endocrine tissues shortly after differentiation. Pancreatic acini are completely devoid of zymogen granules, and the ER is severely distended. Similar ultrastructural alterations are also observed in salivary glands, but not in liver. Accumulation of proteins in the ER lumen activates the proapoptotic pathway of the unfolded protein response, suggesting a central role for apoptosis in the degeneration of these tissues in SEC23B-deficient embryos. Although maintenance of the secretory pathway should be required by all cells, our findings reveal a surprising tissue-specific dependence on SEC23B for the ER exit of highly abundant cargo, with high levels of SEC23B expression observed in professional secretory tissues. The disparate phenotypes in mouse and human could result from residual SEC23B function associated with the hypomorphic mutations observed in humans, or alternatively, might be explained by a species-specific shift in function between the closely related SEC23 paralogues. mammalian embryo abnormalities | vesicular transport protein | genetics | secretory granules | pancreatitis I n eukaryotic cells, secreted proteins and proteins that are targeted to the plasma membrane and internal organelles are synthesized in the endoplasmic reticulum (ER) and sorted through the secretory pathway. This process has been extensively studied, particularly in budding yeast (1). Proteins destined to traffic from the ER to the Golgi are packaged into COPII (coat protein complex II)-coated vesicles (2-4). COPII is composed of at least five proteins, a small GTPase SAR1 and two cytosolic protein complexes, SEC23-SEC24 and SEC13-SEC31 (5). The GTP-bound form of SAR1 binds to the ER membrane and recruits the SEC23-SEC24 heterodimer to form the "prebudding complex," which in turn recruits the outer coat composed of SEC13-SEC31 heterotetramers to complete the COPII coat structure (6).The COPII complex captures cargo into vesicles and mediates vesicle budding from the ER. Cargo recognition appears to be mediated primarily by the SEC24 subunit, which recognizes divergent export signals located in the cytosolic domain of cargo proteins (7,8). SEC23 and SAR1 also play a role in the recognition of at least a subset of cargos (9, 10). SEC23 is a GTPaseactivating protein (GAP) that activates the SAR1...

In yeast, Rad21/Scc1 and its meiotic variant Rec8 are key players in the establishment and subsequent dissolution of sister chromatid cohesion for mitosis and meiosis, respectively, which are essential for chromosome segregation. Unlike yeast, our identification revealed that the rice genome has 4 RAD21-like genes that share lower than 21% identity at polypeptide levels, and each is present as a single copy in this genome. Here we describe our analysis of the function of OsRAD21-4 by RNAi. Western blot analyses indicated that the protein was most abundant in young flowers and less in leaves and buds but absent in roots. In flowers, the expression was further defined to premeiotic pollen mother cells (PMCs) and meiotic PMCs of anthers. Meiotic chromosome behaviors were monitored from male meiocytes of OsRAD21-4-deficient lines mediated by RNAi. The male meiocytes showed multiple aberrant events at meiotic prophase I, including over-condensation of chromosomes, precocious segregation of homologues and chromosome fragmentation. Fluorescence in situ hybridization experiments revealed that the deficient lines were defective in homologous pairing and cohesion at sister chromatid arms. These defects resulted in unequal chromosome segregation and aberrant spore generation. These observations suggest that OsRad21-4 is essential for efficient meiosis.

The type 1-transmembrane protein LMAN1 (ERGIC-53) forms a complex with the soluble protein MCFD2 and cycles between the endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment (ERGIC). Mutations in either LMAN1 or MCFD2 cause the combined deficiency of factor V (FV) and factor VIII (FVIII; F5F8D), suggesting an ER-to-Golgi cargo receptor function for the LMAN1-MCFD2 complex. Here we report the analysis of LMAN1-deficient mice. Levels of plasma FV and FVIII, and platelet FV, are all reduced to ϳ 50% of wild-type in Lman1 ؊/؊ mice, compared with the 5%-30% levels typically observed in human F5F8D patients. Despite previous reports identifying cathepsin C, cathepsin Z, and ␣1-antitrypsin as additional potential cargoes for LMAN1, no differences were observed between wildtype and Lman1 ؊/؊ mice in the levels of cathepsin C and cathepsin Z in liver lysates or ␣1-antitrypsin levels in plasma. LMAN1 deficiency had no apparent effect on COPII-coated vesicle formation in an in vitro assay. However, the ER in Lman1 ؊/؊ hepatocytes is slightly distended, with significant accumulation of ␣1-antitrypsin and GRP78. An unexpected, partially penetrant, perinatal lethality was observed for Lman1 ؊/؊ mice, dependent on the specific inbred strain genetic background, suggesting a potential role for other, as yet unidentified LMAN1-dependent cargo proteins. (Blood. 2011;118(12):3384-3391)

f Congenital dyserythropoietic anemia type II (CDAII) is an autosomal recessive disease of ineffective erythropoiesis characterized by increased bi/multinucleated erythroid precursors in the bone marrow. CDAII results from mutations in SEC23B. The SEC23 protein is a core component of coat protein complex II-coated vesicles, which transport secretory proteins from the endoplasmic reticulum to the Golgi apparatus. Though the genetic defect underlying CDAII has been identified, the pathophysiology of this disease remains unknown. We previously reported that SEC23B-deficient mice die perinatally, exhibiting massive pancreatic degeneration, with this early mortality limiting evaluation of the adult hematopoietic compartment. We now report that mice with SEC23B deficiency restricted to the hematopoietic compartment survive normally and do not exhibit anemia or other CDAII characteristics. We also demonstrate that SEC23B-deficient hematopoietic stem cells (HSC) do not exhibit a disadvantage at reconstituting hematopoiesis when compared directly to wild-type HSC in a competitive repopulation assay. Secondary bone marrow transplants demonstrated continued equivalence of SEC23B-deficient and WT HSC in their hematopoietic reconstitution potential. The surprising discordance in phenotypes between SEC23B-deficient mice and humans may reflect an evolutionary shift in SEC23 paralog function and/or expression, or a change in a specific COPII cargo critical for erythropoiesis.