Characterization of Nonjunctional Hemichannels in Caterpillar Cells

Recent studies have demonstrated that hemichannels, which form gap junctions when paired from apposing cells, may serve additional roles when unpaired including cell adhesion and paracrine communication. Hemichannels in mammals are formed by connexins or pannexins, while in insects they are formed by pannexin homologues termed innexins. The formation of functional gap junctions by insect innexins has been established, although their ability to form functional nonjunctional hemichannels has not been reported. Here the characteristics of nonjunctional hemichannels were examined in three lepidopteran cell types, two cell lines (High Five and Sf9) and explanted hemocytes from Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae). Selective fluorescent dye uptake by hemichannels was observed in a significant minority of cells, using fluorescence microscopy and flow cytometry. Carbenoxelone, an inhibitor of mammalian junctions, disrupted dye uptake, while flufenamic acid and mefloquine did not. The presence of Ca2+ and Mg2+ in the media increased hemichannel activity. Additionally, lipopolysaccharide, a stimulator of immune activity in lepidopterans, decreased dye uptake. These results demonstrate for the first time the activity of nonjunctional hemichannels in insect cells, as well as pharmacological tools to manipulate them. These results will facilitate the further examination of the role of innexins and nonjunctional hemichannels in insect cell biology, including paracrine signaling, and comparative studies of mammalian pannexins and insect innexins

Dynein and kinesin share an overlapping microtubule-binding site

Dyneins and kinesins move in opposite directions on microtubules. The question of how the same-track microtubules are able to support movement in two directions remains unanswered due to the absence of details on dynein–microtubule interactions. To address this issue, we studied dynein–microtubule interactions using the tip of the microtubule-binding stalk, the dynein stalk head (DSH), which directly interacts with microtubules upon receiving conformational change from the ATPase domain. Biochemical and cryo-electron microscopic studies revealed that DSH bound to tubulin dimers with a periodicity of 80 Å, corresponding to the step size of dyneins. The DSH molecule was observed as a globular corn grain-like shape that bound the same region as kinesin. Biochemical crosslinking experiments and image analyses of the DSH–kinesin head–microtubule complex revealed competition between DSH and the kinesin head for microtubule binding. Our results demonstrate that dynein and kinesin share an overlapping microtubule-binding site, and imply that binding at this site has an essential role for these motor proteins

Subunit Structure and Conformations of Tubulin Protofilaments

Microtubules are structural elements of eukaryotic cells which are important for cell motility, cell shape, and cytoplasmic transport (for a review see Kirschner, 1978). One of their characteristic features is the rapid assembly when the cell needs them for a specific purpose such as mitosis, and their disappearance after use. To understand their function one would therefore like to know not only the structure of microtubules but also that of microtubule precursors which might shed light on the process of assembly. The overall shape of flagellar microtubules has been studies by X-ray diffraction, electron microscopy and image proeessing techniques (Grimstone and Klug, 1966; Cohen et al., 1971; Tilney et al., 1973; Arnos and Klug, 1974; and others). They are hollow cylinders of 11 nm mean radius, consisting of 13 protofilaments parallel to the tubule axis. The protofilaments are made up of tubulin monomers of molecular weight 55,000, paired into heterodimers of alpha and beta tubulin. The axial repeat of monomers is 4 nm, their stagger in adjacent protofilaments is about 1 nm