The intramembrane protease SPP impacts morphology of the endoplasmic reticulum by triggering degradation of morphogenic proteins

Abstract

The endoplasmic reticulum (ER), as a multifunctional organelle, plays crucial roles in lipid biosynthesis and calcium homeostasis as well as the synthesis and folding of secretory and membrane proteins. Therefore, it is of high importance to maintain ER homeostasis and to adapt ER function and morphology to cellular needs. Here, we show that signal peptide peptidase (SPP) modulates the ER shape through degradation of morphogenic proteins. Elevating SPP activity induces rapid rearrangement of the ER and formation of dynamic ER clusters. Inhibition of SPP activity rescues the phenotype without the need for new protein synthesis, and this rescue depends on a pre-existing pool of proteins in the Golgi. With the help of organelle proteomics, we identified certain membrane proteins to be diminished upon SPP expression and further show that the observed morphology changes depend on SPP-mediated cleavage of ER morphogenic proteins, including the SNARE protein syntaxin-18. Thus, we suggest that SPP-mediated protein abundance control by a regulatory branch of ER-associated degradation (ERAD-R) has a role in shaping the early secretory pathway.

Regulating the regulator: intramembrane proteolysis of vesicular trafficking proteins and the SERCA regulator phospholamban

“…This work is complemented by a recent publication showing overexpression of a different ER intramembrane protease, signal peptide peptidase (SPP), leads to modulation of ER shape. Organelle SILAC proteomic methods identified the SNARE protein syntaxin 18 as a substrate of the ER localized SPP [6]. This suggests SPPs and SPPLs in similar compartments carry out distinct roles, yet they both contribute to the fidelity of intracellular vesicle trafficking.…”

Physiological functions of SPP/SPPL intramembrane proteases

“…In contrast to SPPL2c which may change Golgi morphology, in cells overexpressing SPP especially the ER is affected. SPP, but not its catalytically-inactive mutant, induces a re-organization of the ER characterized by the formation of densely packed, but highly dynamic ER clusters [52]. This is associated with a suppression of ER-microtubule interactions.…”

“…The access of cytosolic proteins and also of translating ribosomes to the ER cisternae in the centres of the clusters is impaired. Nevertheless, the functionality of the secretory pathway appears to be largely preserved [52]. Despite these striking effects of SPP overexpression on ER morphology, it is unclear yet, to what extent endogenous SPP activity is required to maintain or regulate organisation of the ER.…”

“…However, the precise function of SPP in this process has not been clarified. Since it was shown that SPP is capable of cleaving SNARE proteins [52], it is tempting to speculate that altered trafficking in the early secretory pathway might be involved.…”

Intramembrane proteolysis at a glance: from signalling to protein degradation

“…Another variation of this two-edged function is the role of SPP as a non-canonical factor of the ER-associated degradation (ERAD) pathway (Christianson and Ye, 2014). Although most ERAD substrates are displaced from the ER to the cytoplasm by a dislocon complex, which consists of an ERAD E3 ubiquitin ligase and several auxiliary factors (Wu and Rapoport, 2018), SPP-mediated proteolysis directly triggers the release and degradation of several homeostatic ER-membrane proteins, including the unspliced form of the unfolded protein response regulator XBP1 (Chen et al, 2014), heme oxygenase 1 (Boname et al, 2014) and the SNARE protein syntaxin-18 (Avci et al, 2019). For the purpose of this regulated abundance control, SPP is part of a 500 kDa ERAD complex that consists of the E3 ligase translocation in renal carcinoma on chromosome 8 (TRC8, also known as RNF139) and the rhomboid pseudoprotease Derlin1 (Boname et al, 2014;Chen et al, 2014;Stagg et al, 2009).…”