The beta subunit of the signal recognition particle receptor is a transmembrane GTPase that anchors the alpha subunit, a peripheral membrane GTPase, to the endoplasmic reticulum membrane.

The signal recognition particle receptor (SR) is required for the cotranslational targeting of both secretory and membrane proteins to the endoplasmic reticulum (ER) membrane. During targeting, the SR interacts with the signal recognition particle (SRP) which is bound to the signal sequence of the nascent protein chain. This interaction catalyzes the GTP-dependent transfer of the nascent chain from SRP to the protein translocation apparatus in the ER membrane. The SR is a heterodimeric protein comprised of a 69-kD subunit (SR alpha) and a 30-kD subunit (SR beta) which are associated with the ER membrane in an unknown manner. SR alpha and the 54-kD subunits of SRP (SRP54) each contain related GTPase domains which are required for SR and SRP function. Molecular cloning and sequencing of a cDNA encoding SR beta revealed that SR beta is a transmembrane protein and, like SR alpha and SRP54, is a member of the GTPase superfamily. Although SR beta defines its own GTPase subfamily, it is distantly related to ARF and Sar1. Using UV cross-linking, we confirm that SR beta binds GTP specifically. Proteolytic digestion experiments show that SR alpha is required for the interaction of SRP with SR. SR alpha appears to be peripherally associated with the ER membrane, and we suggest that SR beta, as an integral membrane protein, mediates the membrane association of SR alpha. The discovery of its guanine nucleotide-binding domain, however, makes it likely that its role is more complex than that of a passive anchor for SR alpha. These findings suggest that a cascade of three directly interacting GTPases functions during protein targeting to the ER membrane

⁸⁷Sr/⁸⁶Sr chemostratigraphy of Neoproterozoic Dalradian limestones of Scotland and Ireland: constraints on depositional ages and time scales

New calcite <sup>87</sup>Sr/<sup>86</sup>Sr data for 47 limestones from the metamorphosed and deformed Neoproterozoic-Cambrian Dalradian Supergroup of Scotland and Ireland are used to identify secular trends in seawater <sup>87</sup>Sr/<sup>86</sup>Sr through the Dalradian succession and to constrain its depositional age. Dalradian limestones commonly have Sr greater than 1000 ppm, indicating primary aragonite and marine diagenesis. Low Mn, Mn/Sr less than 0.6, ë<sup>18</sup>O and trace element data indicate that many <sup>87</sup>Sr/<sup>86</sup>Sr ratios are unaltered since diagenesis despite greenschist- to amphibolite-facies metamorphism, consistent with the documented behaviour of Sr and O during metamorphic fluid-rock interaction. Thus, the <sup>87</sup>Sr/<sup>86</sup>Sr data are interpreted largely to reflect <sup>87</sup>Sr/<sup>86</sup>Sr of coeval seawater. Currently available data show that Neoproterozoic seawater <sup>87</sup>Sr/<sup>86</sup>Sr rose from c. 0.7052 at 850-900 Ma to c. 0.7085 or higher in the latest Neoproterozoic. Temporal changes at c. 800 Ma and c . 600 Ma bracket the range in <sup>87</sup>Sr/<sup>86</sup>Sr values of calcite in Grampian. Appin and lowest Argyll Group (c.0.7064-0.7072) and middle and uppermost Argyll Group (c. 0.7082-0.7095) limestones, consistent with a rise in seawater <sup>87</sup>Sr/<sup>86</sup>Sr around 600 Ma. <sup>87</sup>Sr/<sup>86</sup>Sr data are consistent with the sedimentary affinity of the Islay Subgroup with the underlying Appin Group, and with a possible time interval between deposition of Islay and Easdale Subgroup rocks. They indicate that the Dalradian, as a whole, is younger than c. 800 Ma

3D model of amphioxus steroid receptor complexed with estradiol

The origins of signaling by vertebrate steroids are not fully understood. An important advance was the report that an estrogen-binding steroid receptor [SR] is present in amphioxus, a basal chordate with a similar body plan as vertebrates. To investigate the evolution of estrogen binding to steroid receptors, we constructed a 3D model of amphioxus SR complexed with estradiol. This 3D model indicates that although the SR is activated by estradiol, some interactions between estradiol and human ER[alpha] are not conserved in the SR, which can explain the low affinity of estradiol for the SR. These differences between the SR and ER[alpha] in the steroid-binding domain are sufficient to suggest that another steroid is the physiological regulator of the SR. The 3D model predicts that mutation of Glu-346 to Gln will increase the affinity of testosterone for amphioxus SR and elucidate the evolution of steroid binding to nuclear receptors

Evidence for Special Relativity with de Sitter Space-Time Symmetry

I show the formulation of de Sitter Special Relativity (dS-SR) based on Dirac-Lu-Zou-Guo's discussions. dS-SR quantum mechanics is formulated, and the dS-SR Dirac equation for hydrogen is suggested. The equation in the earth-QSO framework reference is solved by means of the adiabatic approach. It's found that the fine-structure "constant" $\alpha$ in dS-SR varies with time. By means of the $t-z$ relation of the $\Lambda$CDM model, $\alpha$'s time-dependency becomes redshift $z$-dependent. The dS-SR's predictions of $\Delta\alpha/\alpha$ agree with data of spectra of 143 quasar absorption systems, the dS-space-time symmetry is SO(3,2) (i.e., anti-dS group) $\;$ and the universal parameter $R$ (de Sitter ratio) in dS-SR is estimated to be $R\simeq 2.73\times 10^{12}ly$. The effects of dS-SR become visible at the cosmic space-time scale (i.e., the distance $\geq 10^9 ly$). At that scale dS-SR is more reliable than Einstein SR. The $\alpha$-variation with time is an evidence of SR with de Sitter symmetry.Comment: 5 pages, 3 figure