Signal Recognition Particle (SRP) and SRP Receptor: A New Paradigm for Multistate Regulatory GTPases

The GTP-binding proteins or GTPases comprise a superfamily of proteins that provide molecular switches in numerous cellular processes. The “GTPase switch” paradigm, in which a GTPase acts as a bimodal switch that is turned “on” and “off” by external regulatory factors, has been used to interpret the regulatory mechanism of many GTPases for more than two decades. Nevertheless, recent work has unveiled an emerging class of “multistate” regulatory GTPases that do not adhere to this classical paradigm. Instead of relying on external nucleotide exchange factors or GTPase activating proteins to switch between the on and off states, these GTPases have the intrinsic ability to exchange nucleotides and to sense and respond to upstream and downstream factors. In contrast to the bimodal nature of the GTPase switch, these GTPases undergo multiple conformational rearrangements, allowing multiple regulatory points to be built into a complex biological process to ensure the efficiency and fidelity of the pathway. We suggest that these multistate regulatory GTPases are uniquely suited to provide spatial and temporal control of complex cellular pathways that require multiple molecular events to occur in a highly coordinated fashion

Dipole-dipole interaction between orthogonal dipole moments in time-dependent geometries

In two nearby atoms, the dipole-dipole interaction can couple transitions with orthogonal dipole moments. This orthogonal coupling accounts for a number of interesting effects, but strongly depends on the geometry of the setup. Here, we discuss several setups of interest where the geometry is not fixed, such as particles in a trap or gases, by averaging over different sets of geometries. Two averaging methods are compared. In the first method, it is assumed that the internal electronic evolution is much faster than the change of geometry, whereas in the second, it is vice versa. We find that the orthogonal coupling typically survives even extensive averaging over different geometries, albeit with qualitatively different results for the two averaging methods. Typically, one- and two-dimensional averaging ranges modelling, e.g., low-dimensional gases, turn out to be the most promising model systems.Comment: 11 pages, 14 figure

Advances in mapping ice-free surfaces within the Northern Antarctic peninsula region using polarimetric RADARSAT-2 data

Ice-free areas within the Northern Antarctic Peninsula region are of interest for studying changes occurring to surface covers, including those related to glacial coverage, raised beach deposits and periglacial processes and permafrost. The objective of this work is to map the main surface covers within ice-free areas of King George Island, the largest island of the South Shetlands archipelago, using fully polarimetric RADARSAT-2 SAR data. Surface covers such as rock outcrops and glacial till, stone fields, patterned ground, and sand and gravel deposits form the most representative classes and account for 84 km2 of the ice-free areas on the island. A distribution of complex geomorphological features and landforms was obtained, being some of them considered indicators of periglacial processes and presence of permafrost.Published versio

La percepción del alemán multietnolectal de Zúrich: un continuo más que una categorización neta

Since about 2000, the emergence of so-called ‘multiethnolects’ has been observed among adolescents in German-speaking Switzerland; however, a systematic description of these varieties is lacking at present. The few existing perception studies of multiethnolects in other European countries are usually based on two or more predetermined groups that are compared. This paper investigates which labels are used for multiethnolectal Zurich German and how this way of speaking is perceived by adolescents; we adopt a perceptual sociolinguistics approach which focuses on the conceptualizations of lay people rather than on those of linguists. In a rating experiment, 40 adolescents listened to short speech samples of 48 pupils recorded in two different schools in the city of Zurich and were asked to rate the speakers on a 7-point Likert scale according to how multiethnolectal they sounded (not at all – very strongly). The results yielded a perceptual continuum rather than a clear-cut binary categorization [±multiethnolectal]. A smaller follow-up experiment with 12 adult raters (using the same stimuli) yielded a highly significant correlation between the mean rating scores of the two groups of raters.A partir del año 2000 aproximadamente se ha observado la aparición de los llamados ‘multietnolectos’ en la Suiza de habla alemana. Sin embargo, hasta el momento no existe una descripción sistemática de estas variedades lingüísticas. Los escasos estudios de percepción que se han realizado en otros países europeos se basan por lo general en la comparación de dos o más grupos preestablecidos. Este artículo investiga qué términos se utilizan para denominar el multietnolecto hablado en Zúrich y cómo un grupo adolescentes califican esta manera de hablar, adoptando, pues, un enfoque de sociolingüística perceptiva que se centra en las representaciones de locutores comunes más que de lingüistas. En un experimento de percepción, cuarenta adolescentes escucharon breves muestras de habla producidas por 48 escolares que habían sido grabadas en dos escuelas de la ciudad de Zúrich. La tarea de los oyentes consistió en calificar a los locutores con una escala de Likert de siete puntos según cómo de multietnolectal sonaba su habla (no en absoluto – muy fuertemente). Los resultados revelan la existencia de un continuum perceptivo más que de una categorización binaria [±multietnolectal]. El mismo experimento se realizó también con ocho oyentes adultos, obteniendo una correlación muy fuerte y altamente significativa con los valores de los oyentes adolescentes. Estos resultados sugieren que en la percepción del multietnolecto alemán de Zúrich no parece existir una diferencia entre una perspectiva etic y una perspectiva emic

Elektrophysiologische Charakterisierung mesenchymaler Stammzellen in myogener Differenzierung

Die stammzellbasierte Regeneration des urethralen Sphinktersystems stellt eine vielversprechende Therapieoption der Belastungsinkontinenz dar. Die hierfür gewählten mesenchymalen Stammzellen wurden zunächst einer in vitro-Differenzierung zu glatten Muskelzellen unterzogen, deren Erfolg neben dem Nachweis muskelspezifischer Proteine auch auf funktioneller Ebene evaluiert wurde. Im Rahmen dieser Arbeit erfolgte die elektrophysiologische Charakterisierung mesenchymaler Stammzellen in myogener Differenzierung mithilfe der manuellen Patch-Clamp-Technik mit dem Ziel der Findung des optimalen Differenzierungszeitpunktes. Die über maximal 21 Tage in Differenzierung befindlichen mesenchymalen Stammzellen wurden im Hinblick auf ihre Expression von Natrium-, Kalium- und Calciumkanälen, sowie ihres Membranpotentials untersucht und mit zuvor erhobenen Daten von nativen mesenchymalen Stammzellen und glatten Muskelzellen des humanen M. detrusor vesicae verglichen. Bereits nach 7 Tagen in Differenzierung wiesen sie eine signifikante Zunahme ihrer Natriumstromdichte auf, die im Verlauf der Differenzierung rückläufig war. Calciumströme wurden ab Differenzierungstag 7 viermal häufiger beobachtet als bei nativen mesenchymalen Stammzellen. Die Veränderungen im Kaliumstromprofil im Vergleich zu nativen mesenchymalen Stammzellen blieben über die Dauer der Differenzierung konstant, unterschieden sich jedoch stark vom Kaliumstromprofil reifer glatter Muskelzellen, was als Hinweis für einen noch ablaufenden Differenzierungsvorgang gewertet wurde. Eine Hyperpolarisation des Membranpotentials gipfelte an Differenzierungstag 7, war statistisch jedoch nicht signifikant. Aus elektrophysiologischer Sicht kann zwar nicht von einer Ausreifung, jedoch zumindest von einer Andifferenzierung der mesenchymalen Stammzellen zu glatten Muskelzellen ausgegangen werden, die um Differenzierungstag 7 ihr Optimum erreicht. Um die funktionelle Untersuchung zu vervollständigen, sollten zusätzlich Methoden zur Überprüfung der Kontraktilität und elektro- und pharmakomechanischen Kopplung der in Differenzierung befindlichen mesenchymalen Stammzellen zum Einsatz kommen

Identification of an adaptor-associated kinase, AAK1, as a regulator of clathrin-mediated endocytosis

The μ2 subunit of the AP2 complex is known to be phosphorylated in vitro by a copurifying kinase, and it has been demonstrated recently that μ2 phosphorylation is required for transferrin endocytosis (Olusanya, O., P.D. Andrews, J.R. Swedlow, and E. Smythe. 2001. Curr. Biol. 11:896–900). However, the identity of the endogenous kinase responsible for this phosphorylation is unknown. Here we identify and characterize a novel member of the Prk/Ark family of serine/threonine kinases, adaptor-associated kinase (AAK)1. We find that AAK1 copurifies with adaptor protein (AP)2 and that it directly binds the ear domain of α-adaptin in vivo and in vitro. In neuronal cells, AAK1 is enriched at presynaptic terminals, whereas in nonneuronal cells it colocalizes with clathrin and AP2 in clathrin-coated pits and at the leading edge of migrating cells. AAK1 specifically phosphorylates the μ subunit in vitro, and stage-specific assays for endocytosis show that μ phosphorylation by AAK1 results in a decrease in AP2-stimulated transferrin internalization. Together, these results provide strong evidence that AAK1 is the endogenous μ2 kinase and plays a regulatory role in clathrin-mediated endocytosis. These results also lend support to the idea that clathrin-mediated endocytosis is controlled by cycles of phosphorylation/desphosphorylation

Differential requirements for AP-2 in clathrin-mediated endocytosis

AP-2 complexes are key components in clathrin-mediated endocytosis (CME). They trigger clathrin assembly, interact directly with cargo molecules, and recruit a number of endocytic accessory factors. Adaptor-associated kinase (AAK1), an AP-2 binding partner, modulates AP-2 function by phosphorylating its μ2 subunit. Here, we examined the effects of adenoviral-mediated overexpression of WT AAK1, kinase-dead, and truncation mutants in HeLa cells, and show that AAK1 also regulates AP-2 function in vivo. WT AAK1 overexpression selectively blocks transferrin (Tfn) receptor and LRP endocytosis. Inhibition was kinase independent, but required the full-length AAK1 as truncation mutants were not inhibitory. Although changes in μ2 phosphorylation were not detected, AAK1 overexpression significantly decreased the phosphorylation of large adaptin subunits and the normally punctate AP-2 distribution was dispersed, suggesting that AAK1 overexpression inhibited Tfn endocytosis by functionally sequestering AP-2. Surprisingly, clathrin distribution and EGF uptake were unaffected by AAK1 overexpression. Thus, AP-2 may not be stoichiometrically required for coat assembly, and may have a more cargo-selective function in CME than previously thought