Autoinhibition and Signaling by the Switch II Motif in the G-protein Chaperone of a Radical B12 Enzyme

MeaB is an accessory GTPase protein involved in the assembly, protection, and reactivation of 5′-deoxyadenosyl cobalamin-dependent methylmalonyl-CoA mutase (MCM). Mutations in the human ortholog of MeaB result in methylmalonic aciduria, an inborn error of metabolism. G-proteins typically utilize conserved switch I and II motifs for signaling to effector proteins via conformational changes elicited by nucleotide binding and hydrolysis. Our recent discovery that MeaB utilizes an unusual switch III region for bidirectional signaling with MCM raised questions about the roles of the switch I and II motifs in MeaB. In this study, we addressed the functions of conserved switch II residues by performing alanine-scanning mutagenesis. Our results demonstrate that the GTPase activity of MeaB is autoinhibited by switch II and that this loop is important for coupling nucleotide-sensitive conformational changes in switch III to elicit the multiple chaperone functions of MeaB. Furthermore, we report the structure of MeaB·GDP crystallized in the presence of AlF_x⁻ to form the putative transition state analog, GDP·AlF₄⁻. The resulting crystal structure and its comparison with related G-proteins support the conclusion that the catalytic site of MeaB is incomplete in the absence of the GTPase-activating protein MCM and therefore unable to stabilize the transition state analog. Favoring an inactive conformation in the absence of the client MCM protein might represent a strategy for suppressing the intrinsic GTPase activity of MeaB in which the switch II loop plays an important role.     

Multiphoton tomography visualizes collagen fibers in the tumor microenvironment that maintain cancer‐cell anchorage and shape

Second harmonic generation (SHG) multiphoton imaging can visualize fibrillar collagen in tissues. SHG has previously shown that fibrillar collagen is altered in various types of cancer. In the present study, in vivo high resolution SHG multi‐photon tomography in living mice was used to study the relationship between cancer cells and intratumor collagen fibrils. Using green fluorescent protein (GFP) to visualize cancer cells and SHG to image collagen, we demonstrated that collagen fibrils provide a scaffold for cancer cells to align themselves and acquire optimal shape. These results suggest a new paradigm for a stromal element of tumors: their role in maintaining anchorage and shape of cancer cells that may enable them to proliferate. J. Cell. Biochem. 114: 99–102, 2012. © 2012 Wiley Periodicals, Inc.     

Physiological mechanisms in the control of bioluminescent countershading in a midwater shrimp

In the oceanic midwater environment, most animals have evolved an extraordinary anti‐predation behavior using bioluminescent countershading (counterillumination) to help them remain cryptic to visual predators. For the midwater penaeid shrimp, Sergestes similis, the interaction of both hormonal and neural systems may be involved in the control of counterillumination. S. similis responds to downward‐directed illumination, detected by the eyes, with light emission from five hepatic light organs. Dark‐adapted specimens undergo a slow induction process prior to production of the conventional counterillumination response. The induction of bio‐luminescence may involve a hormonal pathway mediated by the light‐adapting retinal distal pigment dispersing hormone. Once induced, the rapid control of counterillumination may involve a neural pathway. Because counterilluminating animals directly respond to their optical environment, an understanding of the control of bioluminescence provides an insight into the poorly understood visual processing capabilities of deep‐sea animals.     

Relationships between unitary motor and sensory activity in the walking legs of cancer pagurus (Decapoda,Brachyura)†

Muscular tension and sensory activity in the flexor apodeme sensory nerve were recorded during stimulation of single motor afferents innervating the M‐C flexor. Muscular tension and unitary sensory activity both varied, depending upon the motor fiber stimulated. Differences in the abililty of individual motor fibers to elicit sensory activity were only partially accounted for by differences in tension development. Some tension afferent units were more readily excited by a muscular contraction elicited by one motor axon than they were by another, even when the tension elicited by the more effective motor fiber was less than that evoked by the less effective efferent.     

Computer Simulation of Liquid Crystal Films

Abstract

We present the results of extensive Monte Carlo simulations of liquid crystal films of various thicknesses. A simple nearest-neighbour lattice model, the Lebwohl-Lasher model, is employed, with periodic boundaries in two directions and free, planar, surfaces in the third. Particular attention is devoted to locating the temperature of the order-disorder (nematic-isotropic) phase transition. Weak first-order behaviour apparently persists in systems as thin as 8 layers across, but below this the transition cannot be detected. The shift of the transition temperature from its bulk value approaches the expected asymptotic linear dependence on inverse thickness, but significant deviations from this are seen for films of 10 layers thickness and less. These results enable an accurate estimate to be made of the bulk phase transition temperature in the thermodynamic limit, and the result is consistent with that extrapolated from systems with full periodic boundaries.