ABSTRACT
Posttranslational modification of proteins with the small ubiquitin-related modifier (SUMO) has been implicated in many important physiological functions, including the regulation of transcription and DNA repair. In most cases, only a small fraction of the total cellular amounts of a given protein is sumoylated at a certain point in time. Sensitive detection of sumoylated forms of proteins by western blotting is, therefore, an important step in the identification and/or characterization of a protein control by sumoylation. Polysumoylated proteins are recognized and targeted to the proteasome by specific ubiquitin ligases bearing SUMO interaction motifs. Sumoylation itself is reversible by the action of desumoylating enzymes. Their activities cause a rapid loss of SUMO conjugates in most standard cell extracts. To preserve SUMO-protein conjugates, therefore, a preparation of extracts under denaturing conditions is recommended. Here, we describe the application of an alkaline lysis procedure and a western blot protocol for the analysis of SUMO conjugates in yeast and human cells. In addition, we describe the application of another extraction procedure combined with immobilized metal affinity chromatography for the analysis of ubiquitin-SUMO hybrid conjugates from yeast and human cells.
Subject(s)
Small Ubiquitin-Related Modifier Proteins/analysis , Small Ubiquitin-Related Modifier Proteins/metabolism , Ubiquitin/analysis , Ubiquitin/metabolism , Blotting, Western/methods , Chromatography, Affinity/methods , Humans , Saccharomyces cerevisiae , Small Ubiquitin-Related Modifier Proteins/chemistry , Sumoylation , Ubiquitin/chemistry , Ubiquitin-Conjugating Enzymes/metabolism , Ubiquitin-Protein Ligases/metabolismABSTRACT
We have recently reported that poly-SUMO-2/3 conjugates are subject to a ubiquitin-dependent proteolytic control in human cells. Here we show that arsenic trioxide (ATO) increases SUMO-2/3 modification of promyelocytic leukemia (PML) leading to its subsequent ubiquitylation in vivo. The SUMO-binding ubiquitin ligase RNF4 mediates this modification and causes disruption of PML nuclear bodies upon treatment with ATO. Reconstitution of SUMO-dependent ubiquitylation of PML by RNF4 in vitro and in a yeast trans vivo system revealed a preference of RNF4 for chain forming SUMOs. Polysumoylation of PML in response to ATO thus leads to its recognition and ubiquitylation by RNF4.
Subject(s)
Antineoplastic Agents/pharmacology , Arsenicals/pharmacology , Leukemia, Promyelocytic, Acute/metabolism , Nuclear Proteins/metabolism , Oxides/pharmacology , Small Ubiquitin-Related Modifier Proteins/metabolism , Transcription Factors/metabolism , Ubiquitination/drug effects , Ubiquitins/metabolism , Arsenic Trioxide , Cell Line, Tumor , Down-Regulation , Humans , Proteasome Endopeptidase Complex/metabolismABSTRACT
Posttranslational protein modification with small ubiquitin-related modifier (SUMO) is an important regulatory mechanism implicated in many cellular processes, including several of biomedical relevance. We report that inhibition of the proteasome leads to accumulation of proteins that are simultaneously conjugated to both SUMO and ubiquitin in yeast and in human cells. A similar accumulation of such conjugates was detected in Saccharomyces cerevisiae ubc4 ubc5 cells as well as in mutants lacking two RING finger proteins, Ris1 and Hex3/Slx5-Slx8, that bind to SUMO as well as to the ubiquitin-conjugating enzyme Ubc4. In vitro, Hex3-Slx8 complexes promote Ubc4-dependent ubiquitylation. Together these data identify a previously unrecognized pathway that mediates the proteolytic down-regulation of sumoylated proteins. Formation of substrate-linked SUMO chains promotes targeting of SUMO-modified substrates for ubiquitin-mediated proteolysis. Genetic and biochemical evidence indicates that SUMO conjugation can ultimately lead to inactivation of sumoylated substrates by polysumoylation and/or ubiquitin-dependent degradation. Simultaneous inhibition of both mechanisms leads to severe phenotypic defects.
