The Roll of COMMD1 in Membrane Protein Trafficking

Abstract

Copper is essential for all life due to its role as a catalyst in redox reactions. However, it must be tightly regulated as excess copper can cause oxidative damage to the cell. One example of copper dyshomeostasis is the autosomal recessive condition is known as Wilson disease which can result in liver disease and neurological problems. To maintain copper homeostasis, cells rely on a system of chaperone proteins as well as three copper transporters; CRT1, ATP7A, and ATP7B. ATP7B is a large multidomain membrane‐spanning protein that exports copper from the cytosol and is highly expressed in liver, brain, and kidney cells. ATP7B has a dual role in maintaining copper homeostasis. In the liver, ATP7B functions in the trans‐Golgi network where it pumps copper into the lumen for biosynthesis of secreted cuproproteins. When cellular copper levels are elevated, ATP7B traffics in vesicles towards the apical membrane to move excess copper into bile for excretion from the body. This proves to be a complex decision for the cell as it must retain some ATP7B at the trans‐Golgi for continued biosynthesis of cuproproteins while simultaneously removing the excess. When copper levels are low, ATP7B returns to the Golgi, though it is likely that some is degraded. Although maintaining proper copper homeostasis is important, the mechanism that influences ATP7B distribution is poorly understood. Since being identified as mutated in Bedlington Terriers exhibiting canine copper toxicosis, the protein COMMD1 has been of particular interest as a regulator of cellular copper levels. COMMD1 exhibits both Phosphatidylinositol (PdtIns) effector and Ubiquitin ligase adaptor activity. COMMD1 interacts with a number of membrane proteins including ATP7B. Importantly, COMMD1 downregulation affects cellular copper levels. To investigate the relationship between COMMD1 and ATP7B we used quantitative immunofluorescence microscopy in HepG2 hepatoma cells to analyze the subcellular locations of ATP7B in response to misexpression of COMMD1. The overexpression of the COMMD1 T174M mutant, which was first identified in an atypical Wilson disease patient, significantly decreased the Golgi‐associated fraction of ATP7B and colocalization between ATP7B and lysosomes was also reduced during retrograde trafficking. The T174M mutation may affect PtdIns(4,5)P2 binding by COMMD1, thus we believe that PtdIns(4,5)P2 recruits and facilitates a functional interaction between COMMD1 and ATP7B.Support or Funding InformationNational Science Foundation (MCB‐1411890), NIH 8P20GM103395‐12 (JLB Sub‐Project)