The sterol-sensing domain (SSD) directly mediates signal-regulated endoplasmic reticulum-associated degradation (ERAD) of 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase isozyme Hmg2.

The sterol-sensing domain (SSD) is a conserved motif in membrane proteins responsible for sterol regulation. Mammalian proteins SREBP cleavage-activating protein (SCAP) and HMG-CoA reductase (HMGR) both possess SSDs required for feedback regulation of sterol-related genes and sterol synthetic rate. Although these two SSD proteins clearly sense sterols, the range of signals detected by this eukaryotic motif is not clear. The yeast HMG-CoA reductase isozyme Hmg2, like its mammalian counterpart, undergoes endoplasmic reticulum (ER)-associated degradation that is subject to feedback control by the sterol pathway. The primary degradation signal for yeast Hmg2 degradation is the 20-carbon isoprene geranylgeranyl pyrophosphate, rather than a sterol. Nevertheless, the Hmg2 protein possesses an SSD, leading us to test its role in feedback control of Hmg2 stability. We mutated highly conserved SSD residues of Hmg2 and evaluated regulated degradation. Our results indicated that the SSD was required for sterol pathway signals to stimulate Hmg2 ER-associated degradation and was employed for detection of both geranylgeranyl pyrophosphate and a secondary oxysterol signal. Our data further indicate that the SSD allows a signal-dependent structural change in Hmg2 that promotes entry into the ER degradation pathway. Thus, the eukaryotic SSD is capable of significant plasticity in signal recognition or response. We propose that the harnessing of cellular quality control pathways to bring about feedback regulation of normal proteins is a unifying theme for the action of all SSDs.

Metal-induced structural organization and stabilization of the metalloregulatory protein MntR.

MntR is a metalloregulatory protein that helps to modulate the level of manganese in Bacillus subtilis. MntR shows a metal-response profile distinct from other members of the DtxR family of metalloregulatory proteins, which are generally considered to be iron(II)-activated. As part of an ongoing effort to elucidate the mechanism and metal-selectivity of MntR, several biophysical studies on wild-type MntR and two active site mutants, MntR E99C and MntR D8M, have been performed. Using circular dichroism (CD) spectroscopy, the thermal stability of these proteins has been examined in the presence of various divalent metal ions. Fluorescence intensity measurements of 8-anilino-1-naphthalenesulfonic acid (ANS) were monitored to examine the folding of these proteins in the presence of different metal ions. These experiments indicate that MntR undergoes a significant conformational change upon metal binding that results in stabilization of the protein structure. These studies also show that the MntR D8M active site mutation causes a detrimental effect on the metal-responsiveness of this protein. Fluorescence anisotropy experiments have been performed to quantify the extent of metal-activated DNA binding by these proteins to two different cognate recognition sequences. Binding of MntR and MntR E99C to the mntA cognate sequence closely parallels that of the mntH operator, confirming that the proteins bind both sequences with comparable affinity depending on the activating metal ion. Fluorescence anisotropy experiments on MntR D8M indicate significantly impaired DNA binding, providing additional evidence that MntR D8M is a dysfunctional regulator.

DNA-binding and oligomerization studies of the manganese(II) metalloregulatory protein MntR from Bacillus subtilis.

The metalloregulatory protein MntR from Bacillus subtilis acts as a transcriptional regulator of manganese homeostasis. MntR is a member of a subfamily of DtxR-related proteins that perform analogous regulatory functions in a variety of pathogenic organisms. Metal ions activate MntR to bind DNA and repress the transcription of the mntH gene, which encodes for a proton-coupled metal ion transporter. Size-exclusion chromatography and sedimentation equilibrium ultracentrifugation studies show that apo MntR is predominantly a homodimer in solution. Using fluorescence anisotropy measurements, the DNA binding properties of MntR have been examined. In the strict absence of divalent transition metal ions MntR has a low affinity for the mntH control sequence (K(d) > 8.0 microM). However, binding of MntR is stimulated by the presence of Mn(2+) and Cd(2+) to generate high affinity binding with K(d) values of 16.0 and 7.3 nM, respectively. MntR is also shown to bind the mntH control sequence in the presence of other divalent transition metals, including Ni(2+), Cu(2+), and Zn(2+), but with much lower affinity (K(d) approximately 1.3-2.3 microM). The data here demonstrate that differences in metal-activated DNA binding plays a role in the mechanism of manganese(II)-selective transcription factors and that the oligomerization of MntR is metal-independent, which distinguishes this protein from iron(II)-responsive homologues in the DtxR protein family.