The Lamb The Binding Of Isaac
Latrisha Adan
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a Schematic representation of lambda tail interaction with LamB in close/open state. b Cryo-EM structure of bacteriophage lambda tail interacting with LamB: sideview of the composite map of the bacteriophage lambda tail in open conformation after interaction with LamB (left), along with a top-down view (right). This composite map is generated by merging the open state tube map, a locally masked gpJ map, and the gpJ713 with LamB complex map (for visualization purposes only). The figure illustrates a complex composed of the bacteriophage tail and the receptor LamB, including six units of gpM, three units of gpL, three units of gpJ, three units of gpI, and a trimeric LamB.
Collectively, the four maps reveal the distinct transformations in the lambda phage tail before and after infection (Fig. 1a). Upon LamB binding, the bottom part of gpJ exhibits bending. The tube component demonstrates a descent, coupled with an overall shortening of the entire structure.
The central tail fiber of the lambda phage in the C-terminal region of the gpJ protein is constituted by FNIIIs, AHS, CSF, and RBD (Fig. 3a, left). AHS is a helix bundle formed by three α-helices, situated below the FNIIIs, securely anchoring the three FNIII-2 units together from the bottom. The diameter of the tail tube gradually narrows in the region corresponding to the FNIII, eventually closing completely. In the closed state, the longitudinally oriented CSF, a mixed β-sheet prism, exhibits intricate torsion in the strands spanning from above the RBD to below the AHS. From bottom side to top side, its cross-section progressively expands from narrow to wide (Fig. 3a, left). Upon interaction with LamB, the RBD, CSF, and AHS undergo a range of structural rearrangements, from subtle to substantial (Fig. 3a, right, 3b).
a Left: central tail fiber (gpJ) in a closed state, composed of two consecutive FNIIIs (FNIII-1 and FNIII-2), AHS and CSF. Right bottom: central tail fiber different views in an open state. Right top: Top view of FNIII, AHS, and CSF in an open state. Three FNIIIs strings dissociate and reposition, two chains align on one side of the CSF, while the third string migrates to the opposite side. b Comparison of the secondary structure of AHS and CSF in the closed and open states. 841-861 colored in deep blue to show which helixes in the AHS domains transformed into additional β-sheets in open state structure. c Schematic representation of structural changes in RBD, CSF, and AHS. The binding of RBD to LamB triggers an opening at the bottom, which in turn drives a centripetal motion at the top of RBD. The formation of new hydrogen bonds at the base of CSF results in a more compact CSF structure and induces rotation. The rotation angle is amplified as it propagates upwards, leading to a larger twist at the top of CSF. AHS drops off and wraps around the top of CSF.
Bacteriophage lambda has been widely utilized as a model system for studying host recognition and infection trigger mechanisms. Using the newly obtained open structure of the phage lambda tail with LamB, the closed structure of the central tail fiber, and the previously published closed structure15, we suggest a mechanism for lambda phage DNA ejection induced by receptor binding.
In the closed state of phage lambda, HDI and HDIV act as extenders and adaptors of the tail tube, while HDII and HDIII form the bottom portion of the tail, enveloped by FNIIIs. The trimeric plug domains of gpI obstruct the tube, with the C-terminal region of gpH forming a coiled-coil superhelix that interacts with gpI. The AHS and CSF domains within the tail fiber are twisted, and the bottom ends of the three RBDs are in close proximity (Fig. 6, step 1).
In summary, the implications of phage-receptor complex extend beyond the phage itself. The mechanisms of different bacteriophage-host interactions are central to many areas of microbiology and have potential applications in areas such as phage therapy, a promising alternative to antibiotics in the era of increasing antibiotic resistance34. The small changes in the RBD region and the substantial variations in the structures of AHS and FNIIIs in both open and closed states also illustrate a fascinating biological phenomenon.: how a minute binding event can trigger a series of structural changes and transductions within protein parts, thereby instigating a complex restructuring of a biological machine. This restructuring enables a molecular switch between two stable states, allowing the machine to carry out its complete biological function.
In the case of the tail-LamB complex, a trained Topaz model was utilized for automated particle picking, yielding 2,468,342 picked particles. These particles were further processed through bin4 extraction and 2D classification. From these, 358,235 particles were selected for ab-initio model generation. The best model was chosen for local refinement. The selected particles were re-extracted with shifts applied and then locally refined using different masks, resulting in two high-resolution maps focusing on distinct regions.
UCSF Chimera39 was utilized to dock the gpM, gpL, tail tube domains, and FNIIIs domains of gpJ (PDB ID: 8IYK) into the cryo-EM map. The AHS, CSF, and RBD domains of gpJ were constructed de novo using EMBuilder40. Manual modifications to the models were conducted in COOT41. Structure refinements were subsequently executed in real space utilizing PHENIX42. Supplementary Table S1 contains comprehensive information regarding the 3D reconstruction and model refinement. All structural figures were generated using PyMol43 and ChimeraX44.
We thank Tsinghua University Branch of China National Center for Protein Sciences (Beijing) for kindly providing the cryo-EM facility support and the computational facility support on the cluster of Bio-Computing Platform. We appreciate J. Lei and F. Yang for their technical support. This work was supported by grants from the National Natural Science Foundation of China [grant numbers 32371254 & 32171190].
X.G. and J.W. conceived the project. X.G. prepared phage lambda tails, optimized cryo-grid preparation, recorded the cryo-EM data, and processed the cryo-EM data. J.W. built the atomic models. X.G. and J.W. wrote the manuscript.
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Binding of cortisol to plasma proteins was studied in the foetal lamb by equilibrium dialysis at 37 degrees C. At 122 days of pregnancy the mean level of transcortin expressed as cortisol-binding capacity was 28 +/- 6 (S.D.) ng cortisol/ml plasma. During the last 14 days of pregnancy there was a progressive increase in transcortin-binding capacity to 85 +/- 14 ng cortisol/ml plasma. A sharp increase in the concentration of both protein-bound and unbound cortisol was observed over the same period. A rise in the concentration of total cortisol from around 3 to 42 ng/ml was associated with an increase in unbound cortisol from 0-2 to a maximum of 2-1 ng/ml. The concentration of albumin-bound cortisol was approximately equal to that of unbound cortisol. The mean value for the transcortin-cortisol affinity constant was 1-15 x 10(8) l/mol. It is concluded that an increase in transcortin-binding capacity is partly responsible for the prepartum increase of corticosteroid levels observed in normal foetal lambs.
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