Usp25m protease regulates ubiquitin-like processing of TUG proteins to control GLUT4 glucose transporter translocation in adipocytes

Abstract

Insulin stimulates the exocytic translocation of specialized vesicles in adipocytes, which inserts GLUT4 glucose transporters into the plasma membrane to enhance glucose uptake. Previous results support a model in which TUG (ether containing a BX domain forLUT4) proteins trap these GLUT4 storage vesicles at the Golgi matrix and in which insulin triggers endoproteolytic cleavage of TUG to translocate GLUT4. Here, we identify the muscle splice form of Usp25 (Usp25m) as a protease required for insulin-stimulated TUG cleavage and GLUT4 translocation in adipocytes. Usp25m is expressed in adipocytes, binds TUG and GLUT4, dissociates from TUG-bound vesicles after insulin addition, and colocalizes with TUG and insulin-responsive cargoes in unstimulated cells. Previous results show that TUG proteolysis generates the ubiquitin-like protein, TUGUL (for biquitin-ike). We now show that TUGUL modifies the kinesin motor protein, KIF5B, and that TUG proteolysis is required to load GLUT4 onto these motors. Insulin stimulates TUG proteolytic processing independently of phosphatidylinositol 3-kinase. In nonadipocytes, TUG cleavage can be reconstituted by transfection of Usp25m, but not the related Usp25a isoform, together with other proteins present on GLUT4 vesicles. In rodents with diet-induced insulin resistance, TUG proteolysis and Usp25m protein abundance are reduced in adipose tissue. These effects occur soon after dietary manipulation, prior to the attenuation of insulin signaling to Akt. Together with previous data, these results support a model whereby insulin acts through Usp25m to mediate TUG cleavage, which liberates GLUT4 storage vesicles from the Golgi matrix and activates their microtubule-based movement to the plasma membrane. This TUG proteolytic pathway for insulin action is independent of Akt and is impaired by nutritional excess.

The C‐degron pathway eliminates mislocalized proteins and products of deubiquitinating enzymes

“…We previously found that the N‐terminal domain (NTD) of USP1 generated by autocleavage is removed by KLHDC2 (Lin et al , 2018). Comparably to USP1, the FAU, SDE2, and TUG proteins contain an ubiquitin‐like (UBL) domain with the characteristic C‐terminal diGly motif and all these proteins are processed by deubiquitinating enzymes (Dubs) (Olvera & Wool, 1993; Habtemichael et al , 2018; Thakran et al , 2018). Dubs‐mediated cleavage results in their UBL‐harboring NTD fragments ending with a diGly tail (Fig 7A; Olvera & Wool, 1993; Jo et al , 2016; Habtemichael et al , 2018).…”

“…Comparably to USP1, the FAU, SDE2, and TUG proteins contain an ubiquitin‐like (UBL) domain with the characteristic C‐terminal diGly motif and all these proteins are processed by deubiquitinating enzymes (Dubs) (Olvera & Wool, 1993; Habtemichael et al , 2018; Thakran et al , 2018). Dubs‐mediated cleavage results in their UBL‐harboring NTD fragments ending with a diGly tail (Fig 7A; Olvera & Wool, 1993; Jo et al , 2016; Habtemichael et al , 2018). Therefore, we tested the hypothesis that the diGly/C‐degron KLHDC2 pathway eliminates Dubs cleavage products.…”

Thirty sweet years of GLUT4

“…Clathrin heavy chain-22 is particularly interesting because it is expressed in humans but not in rodents. Additionally, TUG and TC10 have been implicated in GLUT4 exocytosis (58,59). Detailed analysis of in vivo models is required to fully understand the role of all these proteins in regulating metabolism.…”

Acylation – A New Means to Control Traffic Through the Golgi

“…Although this had previously been considered exclusively as a post-Golgi process, more recent data make clear that recycled GLUT4 accumulates in a pool of small (∼50 nm diameter) vesicles that reside near the ERGIC and cis -Golgi compartments, in association with TUG, Golgin-160, and ACBD3 (Acyl-CoA Binding domain-containing protein 3, also known as GCP60) (Xu et al, 2011; Bogan, 2012; Orme and Bogan, 2012; Belman et al, 2015). Upon insulin stimulation, these vesicles are mobilized by TUG cleavage, and they are proposed to traffic to the cell surface by an unconventional secretion pathway that bypasses the Golgi stack (Xu et al, 2011; Bogan, 2012; Habtemichael et al, 2018). GLUT4 is S-acylated and the PAT responsible for this modification was recently identified (Du et al, 2017).…”

“…Data also suggest that the accumulation of a pool of small, insulin-responsive vesicles may be impaired during the development of type 2 diabetes, which may result from excess membrane diacylglycerols and sphingolipids (Garvey et al, 1998; Maianu et al, 2001; Czech, 2017; Habtemichael et al, 2018; Petersen and Shulman, 2018). We speculate that these excess lipids might potentially disrupt the palmitoylation-based sorting mechanisms discussed above and thus contribute to attenuated insulin action.…”