Sci. STKE, 12 April 2005
ION CHANNELS Sumoylation at the Plasma Membrane
SUMO is a small protein that is covalently attached to target proteins through a process similar to that for the attachment of ubiquitin. Rajan et al. provide evidence for SUMO-mediated silencing of a potassium leak channel at the plasma membrane, the K2P1 channel. This channel had previously been investigated, and no current was associated with its expression in oocytes. Rajan et al. found the protein was expressed in oocytes at the plasma membrane and that current could be detected if a single lysine residue (Lys274) was mutated. This lysine was located in a variant SUMO modification site, and yeast two-hybrid analysis indicated that the human SUMO-conjugation enzyme Ubc-9 interacted with the C-terminal portion of K2P1. Coexpression of Ubc-9 [tagged with green fluorescent protein (GFP)] and K2P1 in oocytes resulted in the accumulation of Ubc-9 in the animal pole membrane, which is the same location observed for K2P1 (human Ubc-9 alone or endogenous Xenopus Ubc-9 was uniformly distributed at the plasma membrane). SUMO was detected by Western blotting of K2P1 immunoprecipitated from oocytes expressing the channel, and this modification was not present in K2P1 mutated at Lys274 (K274E-K2P1). Coexpression of human SENP-1, an enzyme that removes SUMO from targets, with K2P1 resulted in a detectable potassium current in the oocytes, suggesting that like the nonsumoylatable K274E mutant, SENP-1 was producing a nonsumoylated form of K2P1 in the oocytes. This was confirmed using two assays. First, a glutathione S-transferase (GST) fusion protein with SENP-1 removed SUMO from K2P1 channels immunoprecipitated from the oocytes. Second, membrane patches containing K2P1 channels expressed with SENP-1 were inserted into native oocytes (presumably containing the endogenous sumoylation machinery) and currents were eliminated. This silencing could be removed by insertion of the membrane patches into the SENP-1-expressing oocytes. To verify that the results were not unique to the oocyte expression system, K2P1, K274E-K2P1, or K274R-K2P1 was expressed in COS-7 cells, and only the mutated channels produced potassium currents. Coexpression of SENP-1 and K2P1 also produced potassium currents. Thus, it appears that SUMO has expanded its sphere of influence to include the plasma membrane. What remains to be determined are the stimuli and pathways that control the addition and removal of SUMO from these channels.
S. Rajan, L. D. Plant, M. L. Rabin, M. H. Butler, S. A. Goldstein, Sumoylation silences the plasma membrane leak K+ channel K2P1. Cell 121, 37-47 (2005). [PubMed]
Citation: Sumoylation at the Plasma Membrane. Sci. STKE 2005, tw132 (2005).
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