Cytosolic localization of NADH cytochrome b₅ oxidoreductase (Ncb5or)

Abstract

Edited by Peter BrzezinskiAcyl-CoA desaturation in the endoplasmic reticulum (ER) membrane depends on cytosolic NADH or NADPH, whereas NADPH in the ER lumen is utilized by prereceptor glucocorticoid production. It was assumed that NADH cytochrome b 5 oxidoreductase (Ncb5or) might connect Acyl-CoA desaturation to ER luminal redox. We aimed to clarify the ambiguous compartmentalization of Ncb5or and test the possible effect of stearoyl-CoA on microsomal NADPH level. Amino acid sequence analysis, fluorescence microscopy of GFP-tagged protein, immunocytochemistry, and western blot analysis of subcellular fractions unequivocally demonstrated that Ncb5or, either endogenous or exogenous, is localized in the cytoplasm and not in the ER lumen in cultured cells and liver tissue. Moreover, the involvement of ER-luminal reducing equivalents in stearoyl-CoA desaturation was excluded.

Cytochrome b5 reductases: Redox regulators of cell homeostasis

“…Unlike the other CYB5R family members, CYB5R4 is a soluble protein that localizes on the ER membrane ( 129 ). However, it should be noted that a more recent study failed to validate CYB5R4’s ER localization ( 130 ). The authors demonstrated that CYB5R4 is positioned in the cytosol and they speculated that CYB5R4 is not an integral protein anchored to the ER membrane but can be transiently recruited from the cytosol for fatty acid desaturation ( 130 ).…”

“…However, it should be noted that a more recent study failed to validate CYB5R4’s ER localization ( 130 ). The authors demonstrated that CYB5R4 is positioned in the cytosol and they speculated that CYB5R4 is not an integral protein anchored to the ER membrane but can be transiently recruited from the cytosol for fatty acid desaturation ( 130 ). The condition and mechanism for CYB5R4’s recruitment to the ER remains to be investigated ( 130 ).…”

“…The authors demonstrated that CYB5R4 is positioned in the cytosol and they speculated that CYB5R4 is not an integral protein anchored to the ER membrane but can be transiently recruited from the cytosol for fatty acid desaturation ( 130 ). The condition and mechanism for CYB5R4’s recruitment to the ER remains to be investigated ( 130 ). Given the multidomain structure of CYB5R4, full-length CYB5R4 has been resistant to crystallization; however, high-resolution crystal structures of the individual domains have been discovered ( 55 ).…”

“…Several studies have suggested that the loss of CYB5R4 results in diabetes mellitus as evidenced by mitochondrial dysfunction, disrupted ion channel signaling and iron homeostasis, and the progressive loss of white adipose tissue in the liver ( 130 , 131 ). Notably, knockout of CYB5R4 caused early-onset diabetes in mice, irrespective of peripheral insulin sensitivity ( 132 ).…”

Characterization and Molecular Mechanism of a Novel Cytochrome b5 Reductase with NAD(P)H Specificity from Mortierella alpina

“…However, a new NADH−Cyt b 5 oxidoreductase (Ncb5or) has been confirmed to link endoplasmic reticulum luminal redox (NADPH) to acyl-CoA desaturation and has affinity for NADPH and NADH. 6,7 Based on the research on Mucoromycota, the NADPH supply is one of the key factors for microbial fatty acid synthesis. 8−14 Typically, Cyt b 5 Rs accept the reduced pyridine nucleotide coenzyme NADH as a physiological electron donor, so it is also called NADH-dependent Cyt b 5 R (NCB5R).…”