SUMOylation Protects Endothelial Cell-Expressed Leukocyte-Specific Protein 1 from Ubiquitination-Mediated Proteasomal Degradation and Facilitates Its Nuclear Export.

Leukocyte-specific protein 1 (LSP1) is known as an endothelial gatekeeper because it controls endothelial permeability and transendothelial cell migration, including that of leukocytes and potentially metastatic cancer cells. In endothelial cells, LSP1 is predominantly in the nucleus under resting conditions but translocates to extranuclear compartments upon stimulation with TNF-α. The discrepancy between its predicted molecular weight (~37 kDa) and its observed migration on SDS-PAGE (≥52 kDa), along with its dynamic subcellular distribution, suggests a possible post-translational modification by SUMOylation. To investigate this, we examined endogenous LSP1 in murine primary endothelial cells and overexpressed recombinant LSP1 in murine endothelial (SVEC4-10EE2) and HEK293T cells. Our results demonstrate that LSP1 is SUMOylated by SUMO1, with Ubc9 serving as the conjugating enzyme and SENP1 as the deSUMOylating protease. Site-directed mutagenesis of lysines K270 and K318 abolished SUMOylation, resulting in a marked reduction in LSP1 steady-state levels. This reduction was attributed to enhanced ubiquitination and accelerated proteasomal degradation of LSP1 in the SUMOylation-deficient state. Furthermore, deSUMOylation impaired the TNF-α-induced translocation of LSP1 from the nucleus to extranuclear compartments, particularly the cytoskeleton. In summary, our findings establish that LSP1 is a SUMO1-modified protein. SUMOylation stabilizes LSP1 by preventing proteasomal degradation and is essential for its proper subcellular trafficking in endothelial cells in response to inflammatory stimuli.