RESUMEN
We study the shape and texture of finite domains comprising chiral or achiral molecules carrying tilt, embedded in a two-dimensional surface or membrane, using a combination of simulations and exact variational calculations. We find a variety of shapes and textures including rectangular-shaped domains and a spontaneously broken chiral texture, when the molecules are achiral. We show that chiral tilt domains nucleating in a region of two-phase coexistence repel each other, thereby preventing coalescence and further growth. Our work principally addresses observations of domains in multicomponent giant unilamellar vesicles. It may also be relevant in the study of domains in phospholipid monolayers, nucleating domains of Sm-C* in Sm-A films, and chiral emulsions in Sm-A films, in situations where we can ignore dipolar interactions.
Asunto(s)
Membrana Dobles de Lípidos/química , Fluidez de la Membrana , Membranas Artificiales , Modelos Químicos , Simulación por Computador , Propiedades de Superficie , Tensión SuperficialRESUMEN
We use monoclonal antibodies against synaptic proteins and anterograde tracing with neurobiotin to describe the architecture of the antennal lobes in different castes of two ant species -Camponotus sericeus and Camponotus compressus. The reproductives and worker classes are readily categorized based on size and external morphology. The overall organization of brain neuropile is comparable between castes with differences only in the visual ganglia. Males have a larger fraction of neuropile occupied by the medulla and lobula than females. In the diurnal species, C. sericeus these regions are more highly represented, than in the nocturnal species C. compressus. The most striking differences are in the antennal lobe where males possess a macroglomerulus, which is about ten times larger in volume than the other glomeruli; such a specialization is absent in females. Minor workers possess a significantly larger number of glomeruli than the majors despite the smaller overall volume of the lobe. These caste-specific differences occur mainly within glomerular clusters that receive input from sensory neurons that project in tracts - T4 and T5 - within the antennal nerve. The comparative anatomy of different castes of ants provides an entry point into a future systematic analysis of how divergent brain architectures can arise within a single species.
Asunto(s)
Hormigas/anatomía & histología , Hormigas/ultraestructura , Encéfalo/fisiología , Encéfalo/ultraestructura , Órganos de los Sentidos/ultraestructura , Animales , Femenino , Masculino , Órganos de los Sentidos/fisiología , Caracteres SexualesRESUMEN
The formation of transport carriers (spherical vesicles and tubules) involves membrane budding, growth, and ultimately fission. We propose a mechanism of membrane budding, wherein the tilt and chirality of constituent molecules, confined to a patch of area A, induces buds of approximately 50-100 nm that are comparable to vesicles involved in endocytosis. Because such chiral and tilted lipid molecules are likely to exist in "rafts", we suggest the involvement of this mechanism in generating membrane buds in the clathrin and dynamin-independent, raft-component mediated endocytosis of glycosylphosphatidylinositol-anchored proteins. We argue that caveolae, permanent cell surface structures with characteristic morphology and enriched in raft constituents, are also likely to be formed by this mechanism. Thus, molecular chirality and tilt, and its expression over large spatial scales may be a common organizing principle in membrane budding of transport carriers.
Asunto(s)
Caveolas/química , Caveolas/fisiología , Endocitosis/fisiología , Fluidez de la Membrana/fisiología , Microdominios de Membrana/química , Microdominios de Membrana/fisiología , Modelos Biológicos , Modelos Químicos , Simulación por Computador , Membrana Dobles de Lípidos/química , Membrana Dobles de Lípidos/metabolismo , Modelos Moleculares , Propiedades de SuperficieRESUMEN
Cholesterol and sphingolipid-enriched "rafts" have long been proposed as platforms for the sorting of specific membrane components including glycosyl-phosphatidylinositol-anchored proteins (GPI-APs), however, their existence and physical properties have been controversial. Here, we investigate the size of lipid-dependent organization of GPI-APs in live cells, using homo and hetero-FRET-based experiments, combined with theoretical modeling. These studies reveal an unexpected organization wherein cell surface GPI-APs are present as monomers and a smaller fraction (20%-40%) as nanoscale (<5 nm) cholesterol-sensitive clusters. These clusters are composed of at most four molecules and accommodate diverse GPI-AP species; crosslinking GPI-APs segregates them from preexisting GPI-AP clusters and prevents endocytosis of the crosslinked species via a GPI-AP-selective pinocytic pathway. In conjunction with an analysis of the statistical distribution of the clusters, these observations suggest a mechanism for functional lipid-dependent clustering of GPI-APs.
Asunto(s)
Membrana Celular/metabolismo , Membrana Celular/fisiología , Membrana Celular/ultraestructura , Animales , Células CHO , Células Cultivadas , Colesterol/metabolismo , Cricetinae , Reactivos de Enlaces Cruzados/farmacología , Endocitosis , Transferencia Resonante de Energía de Fluorescencia , Proteínas Fluorescentes Verdes , Procesamiento de Imagen Asistido por Computador , Luz , Metabolismo de los Lípidos , Lípidos/química , Proteínas Luminiscentes/metabolismo , Microdominios de Membrana/química , Modelos Biológicos , Factores de TiempoRESUMEN
We investigate the steady states and dynamical instabilities resulting from "particles" depositing on (fusion) and pinching off (fission) a fluid membrane. These particles could be either small lipid vesicles or isolated proteins. In the stable case, such fusion/fission events suppress long wavelength fluctuations of the membrane. In the unstable case, the membrane shoots out long tubular structures reminiscent of endosomal compartments or folded structures which bear a morphological resemblance to internal membranes of the cell.