Gap junction protein tissue distribution and abundance in the adult brain in Drosophila

Isolation and characterization of the mitochondrial channel, VDAC, from the insect Heliothis virescens

A 31 kDa voltage-dependent anion-selective channel (VDAC) protein was purified from the insect Heliothis virescens (tobacco budworm, denoted TBW) using an alkali extraction and filtration procedure and was characterized by SDS-PAGE, amino acid sequencing, biophysical properties and immunocytochemistry. The N-terminal sequence has highest identity with VDACs from mammals (50-66%) followed by plants (34-41%) and lower eukaryotes (30-34%). Reconstitution in planar phospholipid membranes yielded properties typical of VDACs from other organisms including a single-channel conductance of 4.1 nS (in 1 M KCl), closure in response to positive and negative transmembrane voltage, and a reversal potential of 11.8 mV indicating anion selectivity in the open state. A polyclonal antiserum (R19) raised against gel-purified 31 kDa protein specifically labelled mitochondria and mitochondrial outer membranes in TBW flight muscle by light and electron microscope immunocytochemistry.

Analog of vertebrate anionic sites in blood-brain interface of larval Drosophila

The blood-brain barrier ensures brain function in vertebrates and in some invertebrates by maintaining ionic integrity of the extraneuronal bathing fluid. Recent studies have demonstrated that anionic sites on the luminal surface of vascular endothelial cells collaborate with tight junctions to effect this barrier in vertebrates. We characterize these two analogous barrier factors for the first time on Drosophila larva by an electron-dense tracer and cationic gold labeling. Ionic lanthanum entered into but not through the extracellular channels between perineurial cells. Tracer is ultimately excluded from neurons in the ventral ganglion mainly by an extensive series of (pleated sheet) septate junctions between perineurial cells. Continuous junctions, a variant of the septate junction, were not as efficient as the pleated sheet variety in blocking tracer. An anionic domain now is demonstrated in Drosophila central nervous system through the use of cationic colloidal gold in LR White embedment. Anionic domains are specifically stationed in the neural lamella and not noted in the other cell levels of the blood-brain interface. It is proposed that in the central nervous system of the Drosophila larva the array of septate junctions between perineurial cells is the physical barrier, while the anionic domains in neural lamella are a "charge-selective barrier" for cations. All of these results are discussed relative to analogous characteristics of the vertebrate blood-brain barrier.

Evidence that the 16 kDa proteolipid (subunit c) of the vacuolar H(+)-ATPase and ductin from gap junctions are the same polypeptide in Drosophila and Manduca: molecular cloning of the Vha16k gene from Drosophila

The 16 kDa proteolipid (subunit c) of the eukaryotic vacuolar H(+)-ATPase (V-ATPase) is closely related to the ductin polypeptide that forms the connexon channel of gap junctions in the crustacean Nephrops norvegicus. Here we show that the major protein component of Manduca sexta gap junction preparations is a 16 kDa polypeptide whose N-terminal sequence is homologous to ductin and is identical to the deduced sequence of a previously cloned cDNA from Manduca (Dow et al., Gene, 122, 355–360, 1992). We also show that a Drosophila melanogaster cDNA, highly homologous to the Manduca cDNA, can rescue Saccharomyces cerevisiae, defective in V-ATPase function, in which the corresponding yeast gene, VMA3, has been inactivated. Evidence is presented for a single genetic locus (Vha16) in Drosophila, which in adults at least contains a single transcriptional unit. Taken together, the data suggest that in Drosophila and Manduca, the same polypeptide is both the proteolipid subunit c component of the V-ATPase and the ductin component of gap junctions. The intron/exon structure of the Drosophila Vha16 is identical to that of a human Vha16 gene, and is consistent with an ancient duplication of an 8 kDa domain. A pilot study for gene inactivation shows that transposable P-elements can be easily inserted into the Drosophila ductin Vha16 gene. Although without phenotypic consequences, these can serve as a starting point for generation of null alleles.