Sumoylation Inhibits Cleavage of Sp1 N-terminal Negative Regulatory Domain and Inhibits Sp1-dependent Transcription

Abstract

Sp1 is a ubiquitously expressed transcription factor that binds GC-rich cis elements. Many posttranslational modifications have been implicated in the regulation of Sp1 activity. We now provide evidence for a novel mechanism of Sp1 regulation involving the small ubiquitin-like modifier (SUMO-1). Western blot analysis revealed a high molecular mass Sp1 of 125 kDa that is stabilized by a selective SUMO hydrolase inhibitor and destabilized by a specific SUMO-1 hydrolase. The covalent modification of Sp1 by endogenous SUMO-1 and SUMO-1 that has been fused to green fluorescent protein was demonstrated using transient transfection assays. A high probability sumoylation consensus motif, VK(16)IE(18), is located within the N-terminal negative regulatory domain of Sp1. Either arginine substitution for lysine 16 (Sp1(K16R)) or alanine substitution for glutamic acid 18 (Sp1(E18A)), abrogated Sp1 sumoylation. In vitro SUMO-1 covalently bound affinity-purified GST-Sp1, but not GST-Sp1(K16R). In vivo Sp1 was determined to be N-terminally cleaved, while Sp1(K16R) could not be cleaved indicating that sumoylation and cleavage are coupled through the key regulatory lysine 16. This coupling was evident by the demonstration of an inverse relationship between cellular SUMO-modified Sp1 and N-terminally cleaved Sp1. Compared with Sp1, sumoylation-deficient Sp1(E18A) exhibited enhanced cleavage and was a better transcriptional activator, while constitutively SUMO-1-modified Sp1 was deficient in proteolytic processing and repressed Sp1 transcriptional activity. The repressive effect of sumoylation on Sp1 activity is emphasized through the use of a GAL4 based transactivation assay. A model is proposed defining a mechanism by which sumoylation preserves the integrity of a negative regulatory domain thereby allowing for the inhibition of Sp-dependent transcription.