Title : Chronic alcohol-mediated egress of a subunit of glucosidase ii out of the rough endoplasmic reticulum
The heterodimeric enzyme a-glucosidase II (GII), an important enzyme in chaperone-assisted folding of nascent glycoproteins in the rough endoplasmic reticulum (RER) is comprised of a beta subunit (GIIb) and the catalytic alpha subunit (GIIa). The soluble GIIa is retained in the RER through its interaction with RER membrane associated GIIb subunit. Recently our laboratory showed that GIIb is a novel RNA binding protein as it binds specifically to a cis-acting region in the 3-UTR of NMDAR1 receptor mRNA. Interaction between GIIb and NMDAR1 mRNA increases following chronic ethanol exposure of fetal cortical neurons and cerebral cortex of adult mouse. Levels of GIIb polypeptide increase significantly in chronic ethanol exposed fetal cortical neurons and cerebral cortex of adult mouse. We examined whether increased levels of GIIb protein in ethanol-exposed FCN are differentially distributed between different subcellular fractions. We show that the 80 kDa GIIb protein is present mainly in the membrane fraction (P100 fraction containing RER) of cultured neurons irrespective of the ethanol treatment. GIIb is absent from pure nuclei although it has a nuclear localization signal. The organelle free cytosol (S100) contains immunoreactive GIIb protein with a calculated molecular weight higher than 100 kDa. Serendipitously we found that chronic ethanol exposure results in redistribution of the GIIa subunit from RER lumen to the organelle free cytosol. The cytosolic GIIa subunit is enzymatically active. Reduction of GII activity in RER may result in inappropriate expression/secretion of neuronal proteins in chronic ethanol exposed neurons. We propose that active GIIa in cytosol may contribute to abnormal protein folding seen in certain neuronal pathologies.
Audience take away:
- Can use the information in teaching neuroscience and biochemistry of proteins
- Can associate the information with “unexplained” ER stress and/or retention of proteins in ER;
- can relate to abnormal protein folding in neurodegenerative diseases.