Hampton, R. Y. | bioRxiv Science

bioRxiv2020

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

HMG-CoA reductase (HMGR) undergoes feedback regulated degradation as part of sterol pathway control. Degradation of the yeast HMGR isozyme Hmg2 is controlled by the sterol pathway intermediate GGPP, which causes misfolding of Hmg2 to enhance its ERAD by the HRD pathway. GGPP-dependent reversible misfolding of Hmg2 is remarkably similar to classic allosteric control; we recently labeled this process mallostery to fuse the ideas of misfolding and allostery. We have evaluated the role of the Hmg2 sterol sensing domain (SSD) in mallostery, and the involvement of highly conserved INSIG proteins in SSD function. The SSD is a membrane-embedded motif found in many sterol-related proteins. The Hmg2 SSD was critical for in vivo regulated degradation of Hmg2, and required for mallosteric misfolding of GGPP as studied by in vitro limited proteolysis. The Hmg2 SSD functions in mallostery independently of conserved yeast INSIG proteins. However, this autonomous action of the SSD was modulated by INSIG, thus imposing a second layer of control on Hmg2 regulation. SSD-mediated mallostery occurs prior to HRD dependent ubiquitination, defining a pathway regulation involving SSD-mediated misfolding followed by HRD dependent ubiquitination. GGPP dependent misfolding occurred at a much slower rate in the absence of a functional SSD, indicating that the SSD functions to allow physiologically useful rate of GGPP response, and implying that the SSD is not a binding site for GGPP. We used unresponsive Hmg2 SSD mutants to test the importance of quaternary structure in mallosteric regulation: the presence of a non-responsive Hmg2 mutant strongly suppressed regulation of a co-expressed, normal Hmg2. Finally, we have found that GGPP regulated misfolding occurred in detergent solubilized Hmg2, indicating that the mallosteric response is an intrinsic feature of the Hmg2 multimer. The preserved response of Hmg2 when in micellar solution will allow next-level studies on the structural and biophysical features of this novel fusion of regulation and protein quality control.

cell biology↗

bioRxiv2017

Ligand-regulated entry into the HRD ERAD pathway: The dark side of allostery

HMG-CoA reductase (HMGR) undergoes regulated degradation as part of feedback control of the sterol pathway. In yeast the stability of the Hmg2 isozyme of HMGR is controlled by the 20 carbon isoprenoid geranylgeranyl pyrophosphate (GGPP): increasing levels of GGPP causes more efficient degradation by the HRD pathway, allowing feedback regulation of HMGR. The HRD pathway is a conserved quality control pathway critical for the ER-associated degradation of misfolded ER proteins. We have explored the action of GGPP in HRD-dependent Hmg2 degradation. GGPP was highly potent as a regulatory molecule in vivo, and in vitro, GGPP altered Hmg2 folding at nanomolar concentrations. These effects of GGPP were absent in a variety of stabilized or non-regulated Hmg2 mutants. Consistent with its high potency, the effects of GGPP were highly specific; other closely related molecules were ineffective in altering Hmg2 structure. In fact, two close GGPP analogues, 2F-GGPP and GGSPP were completely inactive at all concentrations tested, and GGSPP was an antagonist of GGPPs effects in vivo and in vitro. The effects of GGPP on Hmg2 structure and degradation were reversed by chemical chaperones, indicating that GGPP caused selective Hmg2 misfolding. These data indicate that GGPP functions in a manner analogous to an allosteric ligand, causing Hmg2 misfolding through interaction with a reversible, specific binding site. Consistent with this, the Hmg2 protein forms mulitmers. We propose that this \"allosteric misfolding,\" or mallostery, may be a widely used tactic of biological regulation, with potential for development of small molecule pharmaceuticals that induce selective misfolding.

cell biology↗