title: The 3T3-L1 adipocyte glycogen proteome.
abstract: BACKGROUND
  Glycogen is a branched polysaccharide of glucose residues, consisting of α-1-4 glycosidic linkages with α-1-6 branches that together form multi-layered particles ranging in size from 30 nm to 300 nm. Glycogen spatial conformation and intracellular organization are highly regulated processes. Glycogen particles interact with their metabolizing enzymes and are associated with a variety of proteins that intervene in its biology, controlling its structure, particle size and sub-cellular distribution. The function of glycogen in adipose tissue is not well understood but appears to have a pivotal role as a regulatory mechanism informing the cells on substrate availability for triacylglycerol synthesis. To provide new molecular insights into the role of adipocyte glycogen we analyzed the glycogen-associated proteome from differentiated 3T3-L1-adipocytes.
  
  
  RESULTS
  Glycogen particles from 3T3-L1-adipocytes were purified using a series of centrifugation steps followed by specific elution of glycogen bound proteins using α-1,4 glucose oligosaccharides, or maltodextrins, and tandem mass spectrometry. We identified regulatory proteins, 14-3-3 proteins, RACK1 and protein phosphatase 1 glycogen targeting subunit 3D. Evidence was also obtained for a regulated subcellular distribution of the glycogen particle: metabolic and mitochondrial proteins were abundant. Unlike the recently analyzed hepatic glycogen proteome, no endoplasmic proteins were detected, along with the recently described starch-binding domain protein 1. Other regulatory proteins which have previously been described as glycogen-associated proteins were not detected, including laforin, the AMPK beta-subunit and protein targeting to glycogen (PTG).
  
  
  CONCLUSIONS
  These data provide new molecular insights into the regulation of glycogen-bound proteins that are associated with the maintenance, organization and localization of the adipocyte glycogen particle.
authors.affiliation: University of Utah School of Medicine, Rm 4C464B SOM, 30 N 1900 E, Salt Lake City, Utah 84132, USA. glendon.parker@uvu.edu.
authors.forename: David
authors.initials: D
authors.lastname: Stapleton
authors.forename: Chad
authors.initials: C
authors.lastname: Nelson
authors.forename: Krishna
authors.initials: K
authors.lastname: Parsawar
authors.forename: Marcelo
authors.initials: M
authors.lastname: Flores-Opazo
authors.forename: Donald
authors.initials: D
authors.lastname: McClain
authors.forename: Glendon
authors.initials: G
authors.lastname: Parker