Hereditary Spastic Paraplegia Protein Spartin Is an FK506-Binding Protein Identified by mRNA Display

Abstract

Here, we used mRNA display to search for proteins that bind to FK506, a potent immunosuppressant drug, and identified spartin, a hereditary spastic paraplegia protein, from a human brain cDNA library. We demonstrated that FK506 binds to the C-terminal region of spartin and thereby inhibits the interaction of spartin with TIP47, one of the lipid droplet-associated proteins. We further confirmed that FK506 inhibits localization of spartin and its binder, an E3 ubiquitin ligase AIP4, in lipid droplets and increases the protein level of ADRP (adipose differentiation-related protein), which is a regulator of lipid homeostasis. These results strongly suggest that FK506 suppresses the proteasomal degradation of ADRP, a substrate of AIP4, by inhibiting the spartin-TIP47 interaction and thereby blocking the localization of spartin and AIP4 in lipid droplets.

Protein Quality Control and Lipid Droplet Metabolism

“…However, an LD-associated E3 ligase that cooperates with these factors and substrates for this putative degradation pathway has yet to be identified. AIP4/5, which are recruited to LDs by spartin (Eastman et al 2009, Edwards et al 2009, Hooper et al 2010, Tokunaga et al 2013, and SMURF1 (Petrasek et al 2019) have been proposed to function as LD-associated E3 ligases. These proteins have not been observed in most proteomic analyses of LD fractions, although their association may be lost during the biochemical isolation of LDs or be limited to specific cell types.…”