Structural basis for the unique biological function of small GTPase RHEB

The small GTPase Rheb displays unique biological and biochemical properties different from other small GTPases and functions as an important mediator between the tumor suppressor proteins TSC1 and TSC2 and the mammalian target of rapamycin to stimulate cell growth. We report here the three-dimensional structures of human Rheb in complexes with GDP, GTP, and GppNHp (5'-(beta,gamma-imide)triphosphate), which reveal novel structural features of Rheb and provide a molecular basis for its distinct properties. During GTP/GDP cycling, switch I of Rheb undergoes conformational change while switch II maintains a stable, unusually extended conformation, which is substantially different from the alpha-helical conformation seen in other small GTPases. The unique switch II conformation results in a displacement of Gln64 (equivalent to the catalytic Gln61 of Ras), making it incapable of participating in GTP hydrolysis and thus accounting for the low intrinsic GTPase activity of Rheb. This rearrangement also creates space to accommodate the side chain of Arg15, avoiding its steric hindrance with the catalytic residue and explaining its noninvolvement in GTP hydrolysis. Unlike Ras, the phosphate moiety of GTP in Rheb is shielded by the conserved Tyr35 of switch I, leading to the closure of the GTP-binding site, which appears to prohibit the insertion of a potential arginine finger from its GTPase-activating protein. Taking the genetic, biochemical, biological, and structural data together, we propose that Rheb forms a new group of the Ras/Rap subfamily and uses a novel GTP hydrolysis mechanism that utilizes Asn1643 of the tuberous sclerosis complex 2 GTPase-activating protein domain instead of Gln64 of Rheb as the catalytic residue.     

Neural basis of mathematical cognition

The human brain has remarkable capabilities for encoding and manipulating information about quantities. Understanding how the brain carries out such number and quantity processing is a problem not just for those interested in numerical cognition: it raises important questions that are relevant to understanding development, action, vision, language, executive function and cortical organisation. It is also a clear case of research into a core human psychological function having indisputable everyday relevance; hence the emphasis in early education on numeracy and later on mathematics.     

The many hues of plant heterochromatin

Recent sequence and cytogenetic analyses of heterochromatin in Arabidopsis, together with other results from Arabidopsis and maize, indicate that plant heterochromatin can have very different origins, compositions and dynamics. Shared features that must determine and/or be a result of its unique biological properties are also revealed.     

Neural basis of the ventriloquist illusion

The ventriloquist creates the illusion that his or her voice emerges from the visibly moving mouth of the puppet [1]. This well-known illusion exemplifies a basic principle of how auditory and visual information is integrated in the brain to form a unified multimodal percept. When auditory and visual stimuli occur simultaneously at different locations, the more spatially precise visual information dominates the perceived location of the multimodal event. Previous studies have examined neural interactions between spatially disparate auditory and visual stimuli [2-5], but none has found evidence for a visual influence on the auditory cortex that could be directly linked to the illusion of a shifted auditory percept. Here we utilized event-related brain potentials combined with event-related functional magnetic resonance imaging to demonstrate on a trial-by-trial basis that a precisely timed biasing of the left-right balance of auditory cortex activity by the discrepant visual input underlies the ventriloquist illusion. This cortical biasing may reflect a fundamental mechanism for integrating the auditory and visual components of environmental events, which ensures that the sounds are adaptively localized to the more reliable position provided by the visual input.     

Neural basis of quasi-rational decision making

Standard economic theories conceive homo economicus as a rational decision maker capable of maximizing utility. In reality, however, people tend to approximate optimal decision-making strategies through a collection of heuristic routines. Some of these routines are driven by emotional processes, and others are adjusted iteratively through experience. In addition, routines specialized for social decision making, such as inference about the mental states of other decision makers, might share their origins and neural mechanisms with the ability to simulate or imagine outcomes expected from alternative actions that an individual can take. A recent surge of collaborations across economics, psychology and neuroscience has provided new insights into how such multiple elements of decision making interact in the brain.     

Neural cells secrete a unique repertoire of proteins

Proteins that are released from cells consist of those in the extracellular matrix, as well as extracellular signaling and adhesion molecules. The majority of these extracellular proteins are, however, unknown. To determine their identity, we have used a proteomics approach to define proteins released from neurons, astrocytes and neural precursor cells. Using two-dimensional gels and liquid chromatography/mass spectrometry technology, it is shown that while astrocytes release a relatively small number of proteins, neurons and neuronal precursor cells release a larger number of proteins with more functional diversity. Although there is overlap between the different cell types, the exact composition of the extracellular protein pool is unique for each cell population. The various subsets of extracellular neural proteins include those involved in cellular Redox regulation and chaperones. In addition, many proteolytic enzymes are found outside of the cell. These data show that the extracellular space within the nervous system has a more diverse protein composition than previously thought.     

Neural basis for successful encoding and retrieval of prospective memory

Prospective memory (PM) refers to memory for future intentions. Difference due to memory (Dm effect) is the difference in neural activity related to stimuli that were subsequently remembered or forgotten. Using event-related potentials (ERPs), the present study investigated the Dm effect for PM using a subsequent task-switching paradigm. The results showed that a Dm effect of ERP P150 was more positive-going for later PM hit trials than for later PM forgotten trials during 100–200 ms. This Dm effect may reflect the process for the production of future intention or the process for attention. Consistent with previously reported Dm effects of other types of memory, we found that the fbN2 (250–280 ms) and late positivity component (400–700 ms) were stronger in later PM hit trials than in forgotten trials. The fbN2 was evoked by Chinese characters. The late positivity component was related to the precise encoding process. In conclusion, because of the early P150, PM encoding appears to be somewhat different from previously identified Dm effects. However, further research is needed. Our findings reveal that Dm effects of PM share similar characteristics with known Dm effects of other types of episodic memory after the very early stage of neural processing.     

成功的前瞻记忆编码与提取的神经机制研究

前瞻记忆是指对将来行为和事件的记忆,而相继记忆效应（Dm效应）是指根据记忆提取阶段的成绩,分析与之对应的编码阶段的项目和忘的项目之间的神经活动的差异.本研究采用任务转化范式,借助ERP仪器,使用汉字材料,探讨了前瞻记忆的Dm效应.研究发现,100～200ms（P150）,250～280ms（fbN2）,400～700ms（LPC）随后前瞻记忆击中的ERPs比随后前瞻记忆忘的ERPs更正.其中,早期的相继记忆效应（P150更正）可能反映了前瞻记忆将来意向的加工或者准备意的加工;而后两个时段与以往的研究结果一致,fbN2可能与汉语材料的加工有关,LPC则反映了记忆的精确编码.总之,本研究从ERP的角度提供了前瞻记忆编码与以往情景记忆编码有相同之处（LPC更正）,又存在一些差异（P150更正）的证据,但这种差异需要进一步的研究来论证.     

Molecular basis for the unique deubiquitinating activity of the NF-kappaB inhibitor A20

Nuclear factor κB (NF-κB) activation in tumor necrosis factor, interleukin-1, and Toll-like receptor pathways requires Lys63-linked nondegradative polyubiquitination. A20 is a specific feedback inhibitor of NF-κB activation in these pathways that possesses dual ubiquitin-editing functions. While the N-terminal domain of A20 is a deubiquitinating enzyme (DUB) for Lys63-linked polyubiquitinated signaling mediators such as TRAF6 and RIP, its C-terminal domain is a ubiquitin ligase (E3) for Lys48-linked degradative polyubiquitination of the same substrates. To elucidate the molecular basis for the DUB activity of A20, we determined its crystal structure and performed a series of biochemical and cell biological studies. The structure reveals the potential catalytic mechanism of A20, which may be significantly different from papain-like cysteine proteases. Ubiquitin can be docked onto a conserved A20 surface; this interaction exhibits charge complementarity and no steric clash. Surprisingly, A20 does not have specificity for Lys63-linked polyubiquitin chains. Instead, it effectively removes Lys63-linked polyubiquitin chains from TRAF6 without dissembling the chains themselves. Our studies suggest that A20 does not act as a general DUB but has the specificity for particular polyubiquitinated substrates to assure its fidelity in regulating NF-κB activation in the tumor necrosis factor, interleukin-1, and Toll-like receptor pathways.     

Molecular basis for a functionally unique cytochrome P450IIB1 variant.

Liver microsomes from phenobarbital-treated rats of four inbred strains expressing distinct allelic variants of cytochrome P450IIB1 were analyzed. The Wistar Munich (WM) strain exhibited 5- to 10-fold lower androstenedione 16 beta-hydroxylase activity (a specific P450IIB1 marker) than the Lewis, Wistar Kyoto, and Wistar Furth strains. The androstenedione 16 beta-hydroxylase in the WM liver microsomes was refractory to inactivation by N-(2-p-nitrophenethyl)chlorofluoroacetamide, a selective P450IIB1 inactivator in the other three strains. Purified P450IIB1-WM was insensitive to the inactivator and exhibited 5-fold lower androstenedione 16 beta-hydroxylase, testosterone 16-hydroxylase, and 7-ethoxycoumarin deethylase activities but the same benzphetamine demethylase activity and slightly higher androstenedione 16 alpha-hydroxylase activity than a P450IIB1 purified from outbred Sprague-Dawley rats, which appears to correspond to the form in Lewis rats. The stereoselectivity of androstenedione 16-hydroxylation catalyzed by P450IIB1-WM (16 beta-OH:16 alpha-OH = 1.4) is thus distinct from that (16 beta-OH:16 alpha-OH = 12-15) of other P450IIB1 preparations described. A cDNA encoding P450IIB1-WM was cloned and sequenced, revealing a single amino acid substitution (Gly-478----Ala) compared with the published sequence (Fujii-Kuriyama, Y., Mizukami, Y., Kawajiri, K., Sogawa, K., and Muramatsu, M. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 2793-2797). Heterologous expression of P450IIB1 and P450IIB1-WM confirmed the striking difference in androstenedione metabolite profiles, strongly implicating the involvement of Ala-478 in defining the distinctive catalytic properties of P450IIB1-WM.     

Neural basis of deciding, choosing and acting

The ability and opportunity to make decisions and carry out effective actions in pursuit of goals is central to intelligent life. Recent research has provided significant new insights into how the brain arrives at decisions, makes choices, and produces and evaluates the consequences of actions. In fact, by monitoring or manipulating specific neurons, certain choices can now be predicted or manipulated.     

A structural basis for the unique binding features of the human vitamin D-binding protein.

The human serum vitamin D-binding protein (DBP) has many physiologically important functions, ranging from transporting vitamin D3 metabolites, binding and sequestering globular actin and binding fatty acids to functioning in the immune system. Here we report the 2.3 A crystal structure of DBP in complex with 25-hydroxyvitamin D3, a vitamin D3 metabolite, which reveals the vitamin D-binding site in the N-terminal part of domain I. To more explicitly explore this, we also studied the structure of DBP in complex with a vitamin D3 analog. Comparisons with the structure of human serum albumin, another family member, reveal a similar topology but also significant differences in overall, as well as local, folding. These observed structural differences explain the unique vitamin D3-binding property of DBP.     

Speciation genetics: reinforcement by shades and hues

Mating with a member of another species can seriously reduce an organism's fitness, so mechanisms ought to evolve to prevent it where hybridizing species meet. This old idea of 'reinforcement' has found new support in an elegant pair of studies of the ecological genetics of flower colour in an annual herb.