Proteostatic Tactics in the Strategy of Sterol Regulation

Abstract

In eukaryotes, the synthesis and uptake of sterols undergo stringent multivalent regulation. Both individual enzymes and transcriptional networks are controlled to meet changing needs of the many sterol pathway products. Regulation is tailored by evolution to match regulatory constraints, which can be very different in distinct species. Nevertheless, a broadly conserved feature of many aspects of sterol regulation is employment of proteostasis mechanisms to bring about control of individual proteins. Proteostasis is the set of processes that maintain homeostasis of a dynamic proteome. Proteostasis includes protein quality control pathways for the detection, and then the correction or destruction, of the many misfolded proteins that arise as an unavoidable feature of protein-based life. Protein quality control displays not only the remarkable breadth needed to manage the wide variety of client molecules, but also extreme specificity toward the misfolded variants of a given protein. These features are amenable to evolutionary usurpation as a means to regulate proteins, and this approach has been used in sterol regulation. We describe both well-trod and less familiar versions of the interface between proteostasis and sterol regulation and suggest some underlying ideas with broad biological and clinical applicability.

An autonomous, but INSIG-modulated, role for the Sterol Sensing Domain in mallostery-regulated ERAD of yeast HMG-CoA reductase

“…The SSD is a multispanning membrane motif found in a number of eukaryotic proteins involved in sterol synthesis, transport, and regulation. The demonstrated importance of the Hmg2 SSD in GGPP-mediated misfolding was consistent with a broad role of SSD in mediating protein structural changes as a mechanism of metabolic regulation (13,16,40). Importantly, our results demonstrate an autonomous role for a bone fide SSD in protein regulation independent of the frequently described INSIG-SSD interactions often required for SSD actions.…”

“…The sterol-sensing domain (SSD) is a motif found in the multispanning membrane domains of many proteins that function in sterol synthesis, response, or regulation, including yeast Hmg2 (13,(19)(20)(21)(22). This highly conserved motif occurs in the intra-and juxta-membrane residues over five adjacent membrane-spanning alpha helices.…”

“…The SSD is perhaps best known for mediating the sterol-dependent binding to the regulatory proteins known as INSIGs (insulin-stimulated genes). In many cases, an SSD-INSIG interaction underlies regulation that requires an SSD (13,(29)(30)(31)(32)(33). For example, mammalian HMGcR requires SSD-mediated INSIG binding for sterol-regulated degradation (29,30).…”

“…Our studies have revealed that the high selectivity of the HRD pathway for misfolded versions of substrates underlies HRD-dependent regulation of Hmg2 stability by GGPP: when GGPP levels are elevated, Hmg2 undergoes a reversible structural transition to a more misfolded form, allowing enhanced recognition and destruction by the HRD pathway and consequent lowering of Hmg2 activity. It appears that this mode of regulation by quality control has many examples throughout biology (9)(10)(11)(12)(13)(14)(15)(16) .…”

“…vivo--no conserved residue mutants resulted in decreased stability--suggesting that the normal function of SSD is to allow regulated misfolding to occur. Because of the near-universal involvement of INSIGs in the action of SSDs in cholesterol regulation(13,29,(31)(32)(33), we tested whether the SSD-dependent structural change was intrinsic to the domain itself, or rather required the yeast INSIG orthologs Nsg1 or Nsg2. Despite the expectation for an INSIG role, we found that neither INSIG paralog was required for SSD-dependent GGPP-regulated misfolding.However, consistent with our earlier studies(31,32), coexpressing Nsg1 to levels matching Hmg2 actually blocked regulated in vitro misfolding and in vivo degradation.…”