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At least 829 records · Page 46Linked to original sources
PubMed1998

Breaking the camel's back: proline-induced turns in a model transmembrane helix.

We have constructed model membrane proteins with hydrophobic segments of the general composition Leu29Val-Leun where n=10 and 20, and have analyzed their transmembrane topology when inserted into microsomal membranes. These hydrophobic segments span the membrane once, even though they are twice as long as normal transmembrane helices. Strikingly, a single proline residue introduced near the center of the Leu39Val hydrophobic stretch induces the formation of two transmembrane segments separated by a tight turn. These results have implications for our understanding of membrane protein assembly in the endoplasmic reticulum, and for the development of techniques for predicting membrane protein topology.

Amino Acid Sequence↗
PubMed1975

Male meiosis in camel-flies (Raphidioptera; Neuropteroidea).

Male meiosis in 3 species of the raphidioptera genus Agulla-- A. bicolor Banks, A. astuta (Banks), and A. bractea Carpenter-- closely parallels that of Neuroptera. The diploid complement in each comprises 12 pairs of autosomes plus X and Y; all are mediokinetic. One male of A. bicolor carried an extra pair of autosomes indistinguishable from the shortest member of the usual set: these formed a normal bivalent and segregated synchronously with the other autosomes. The spindle is formed by the collocation of individual units which envelope each chromosomal mass. The sex chromsomes are spatially separate on emergence from the joint vesicle of early prophase; oriented toward opposite poles they move into this interpolar axis and a central spindle unit forms about them. This unit elongates disproportionately in early premetaphase, and its subsequent contraction is not synchronous with that of the other units. Distance segregation of X and Y is completed in early premetaphase. Autosomal bivalents are chiasmate; their congressional maneuvers involve, in addition to the usual interpolar oscillations, a lateral movement to the periphery of the spindle to form a variably complete ring at the equator. Autosomal univalents occurs with a frequency of 13% in A. bicolor, 2% in A. astuta, and 1% in A. bractea; they undergo distance segregation with the sex chromosomes in the central spindle unit. The phylogenetic significance of the data is considered.

Animals↗
PubMed1989

Effect of blood meal and mating on the genital tract ultrastructure in the female camel tick Hyalomma (Hyalomma) dromedarii (Ixodoidea: Ixodidae).

The genital tract ultrastructure in the female Hyalomma (Hyalomma) dromedarii is described during feeding and mating and up to oviposition. The vagina, consisting of vestibular (VV) and cervical (CV) regions, is formed of an epithelium lined internally with a folded cuticular layer and surrounded externally by muscle layers. These facilitate the passage of endospermatophores containing sperm into the receptaculum seminis (RS), and ova to the exterior. A pair of tubular accessory glands (AG) opening at the junction of VV and CV consist of an epithelial layer of undifferentiated cells. As feeding progresses, these cells synthesise their granular secretion which lubricates the egg surface during its passage through VV. The RS, opening anteriorly into the CV, consists of cuboidal cells lined with a thin cuticular layer. These cells become rich in glycogen and lipid vacuoles, possibly acting as a source of energy for various cell activities including granule synthesis and exocytosis. The granules discharge their contents into intercellular spaces distributed throughout the RS wall and communicate with the main lumen via narrow channels. The cell secretion may dissolve the endospermatophore wall to release sperm, while their lysosome-like structures may function in the breakdown of endospermatophoric material taken up by pinocytosis. The connecting tube (CT) opens into the CV anterodorsally and leads into the common oviduct (COV) posteriorly. It consists of an epithelium lined by a closely-adhering cuticular layer, giving the tube lumen the appearance of an undulate labyrinth with a complicated configuration. No secretory activity in the CT has been observed before, during, or after feeding. The paired, non-cuticular oviducts, extending from the ovary and fusing anteriorly to form COV, consist of an epithelium poor in cell organelles. At the final stages of feeding the cell cytoplasm contains large, phagosomal vacuoles penetrated by sperms, in addition to micropinocytotic vesicles which serve to break down the seminal fluid and other materials. The basal membrane is infolded giving characteristic features, which increase dramatically during oviposition, of epithelia involved in ion and water transport. The oviducal secretion may function as a tanning agent to harden the egg-shell and may also act as a lubricant for egg passage.

Animals↗