SPRING licenses S1P-mediated cleavage of SREBP2 by displacing an inhibitory pro-domain.

Site-one protease (S1P) conducts the first of two cleavage events in the Golgi to activate Sterol regulatory element binding proteins (SREBPs) and upregulate lipogenic transcription. S1P is also required for a wide array of additional signaling pathways. A zymogen serine protease, S1P matures through autoproteolysis of two pro-domains, with one cleavage event in the endoplasmic reticulum (ER) and the other in the Golgi. We recently identified the SREBP regulating gene, (SPRING), which enhances S1P maturation and is necessary for SREBP signaling. Here, we report the cryo-EM structures of S1P and S1P-SPRING at sub-2.5 Å resolution. SPRING activates S1P by dislodging its inhibitory pro-domain and stabilizing intra-domain contacts. Functionally, SPRING licenses S1P to cleave its cognate substrate, SREBP2. Our findings reveal an activation mechanism for S1P and provide insights into how spatial control of S1P activity underpins cholesterol homeostasis.

Identification of a degradation signal at the carboxy terminus of SREBP2: A new role for this domain in cholesterol homeostasis.

Lipid homeostasis in animal cells is maintained by sterol regulatory element-binding proteins (SREBPs), membrane-bound transcription factors whose proteolytic activation requires the cholesterol-sensing membrane protein Scap. In endoplasmic reticulum (ER) membranes, the carboxyl-terminal domain (CTD) of SREBPs binds to the CTD of Scap. When cholesterol levels are low, Scap escorts SREBPs from the ER to the Golgi, where the actions of two proteases release the amino-terminal domains of SREBPs that travel to the nucleus to up-regulate expression of lipogenic genes. The CTD of SREBP remains bound to Scap but must be eliminated so that Scap can be recycled to bind and transport additional SREBPs. Here, we provide insights into how this occurs by performing a detailed molecular dissection of the CTD of SREBP2, one of three SREBP isoforms expressed in mammals. We identify a degradation signal comprised of seven noncontiguous amino acids encoded in exon 19 that mediates SREBP2's proteasomal degradation in the absence of Scap. When bound to the CTD of Scap, this signal is masked and SREBP2 is stabilized. Binding to Scap requires an arginine residue in exon 18 of SREBP2. After SREBP2 is cleaved in Golgi, its CTD remains bound to Scap and returns to the ER with Scap where it is eliminated by proteasomal degradation. The Scap-binding motif, but not the degradation signal, is conserved in SREBP1. SREBP1's stability is determined by a degradation signal in a different region of its CTD. These findings highlight a previously unknown role for the CTD of SREBPs in regulating SREBP activity.

Sex-Specific Modulation of Blood Pressure and the Renin-Angiotensin System by ACE (Angiotensin-Converting Enzyme) 2.

We showed ACE (angiotensin-converting enzyme) 2 is higher in the kidney of male compared with female mice. To further investigate this sex difference, we examined the role of ACE2 in Ang-[1-8] (angiotensin [1-8])-induced hypertension and regulation of the renin-angiotensin system in the kidney of WT (wild type) and Ace2 KO (knockout) mice. Mean arterial pressure rose faster in WT male than WT female mice after Ang-[1-8] infusion. This sex difference was attenuated in ACE2 KO mice. Ang-[1-8] infusion reduced glomerular AT1R (angiotensin type 1 receptor) binding in WT female mice by 30%, and deletion of Ace2 abolished this effect. In contrast, Ang-[1-8] infusion increased glomerular AT1R binding in WT male mice by 1.2-fold, and this effect of Ang-[1-8] persisted in Ace2 KO male mice (1.3-fold). ACE2 also had an effect on renal protein expression of the neutral endopeptidase NEP (neprilysin), the enzyme that catabolizes Ang-[1-10] (angiotensin [1-10]), the precursor of Ang-[1-8]. Ang-[1-8] infusion downregulated NEP protein expression by 20% in WT male, whereas there was a slight increase in NEP expression in WT female mice. Deletion of Ace2 resulted in lowered NEP expression after Ang-[1-8] infusion in both sexes. These findings suggest sex-specific ACE2 regulation of the renin-angiotensin system contributes to female protection from Ang-[1-8]-induced hypertension. These findings have ramifications for the current coronavirus disease 2019 (COVID-19) pandemic, especially in hypertension since ACE2 is the SARS-CoV-2 receptor and hypertension is a major risk factor for poor outcomes.