Nuclear import of glucokinase in pancreatic beta-cells is mediated by a nuclear localization signal and modulated by SUMOylation

Publication date: Available online 22 June 2017
Source:Molecular and Cellular Endocrinology
Author(s): Bente Berg Johansson, Karianne Fjeld, Marie Holm Solheim, Jun Shirakawa, Enming Zhang, Magdalena Keindl, Jiang Hu, Andreas Lindqvist, Anne Døskeland, Gunnar Mellgren, Torgeir Flatmark, Pål Rasmus Njølstad, Rohit N. Kulkarni, Nils Wierup, Ingvild Aukrust, Lise Bjørkhaug
The localization of glucokinase in pancreatic beta-cell nuclei is a controversial issue. Although previous reports suggest such a localization, the mechanism for its import has so far not been identified. Using immunofluorescence, subcellular fractionation and mass spectrometry, we here present evidence in support of glucokinase localization in beta-cell nuclei of human and mouse pancreatic sections, as well as in human and mouse isolated islets, and murine MIN6 cells. We have identified a conserved, seven-residue nuclear localization signal (³⁰LKKVMRR³⁶) in the human enzyme. Substituting the residues KK^31,32 and RR^35,36 with AA led to a loss of its nuclear localization in transfected cells. Furthermore, our data indicates that SUMOylation of glucokinase modulates its nuclear import, while high glucose concentrations do not significantly alter the enzyme nuclear/cytosolic ratio. Thus, for the first time, we provide data in support of a nuclear import of glucokinase mediated by a redundant mechanism, involving a nuclear localization signal, and which is modulated by its SUMOylation. These findings add new knowledge to the functional role of glucokinase in the pancreatic beta-cell.



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