Genetic and functional analysis of tryptophan transport in Malpighian tubules of Drosophila

Abstract

Dissected Malpighian tubules from wild type and the eye color mutant white of Drosophila were compared with respect to their abilities to transport tryptophan and kynurenine into tubule cells. It was determined that mutation at white greatly impairs the ability of Malpighian tubule cells to take up tryptophan. Functional studies on the extracellular spaces and ultrastructural observations indicated no differences in these respects between wild type and white tubules. It is consistent with several observations that much of the tryptophan associated with white exists in the intercellular spaces. Furthermore, the uptake of tryptophan by the w+ system of wild type tubules is inhibited by the analogue 5-methyl-tryptophan. However, the incorporation of radioactive tryptophan into protein in tubule cells from wild type and white occurs at the same rates and is not affected by 5-methyl-tryptophan. Therefore, it is apparent that Malpighian tubules have a transport system that enables entry of tryptophan into a cellular pool and that this cellular pool is initially independent of the tryptophan pool used for protein synthesis. The mutant white lacks this transport system. From these studies and others it appears that compartmentalization of cellular pools may be brought about via the utilization of specific membrane transport systems.

A new role for a classical gene: White transports cyclic GMP

“…To initiate these studies, potential substrates of White, including tryptophan, kynurenine and 3-hydroxykynurenine (Howells et al, 1977;Sullivan et al, 1980) were utilised in the cGMP transport competition assays (Table·3). At high concentrations, kynurenine inhibited both cGMP and cAMP transport (Table·3), but tryptophan did not.…”

“…Investigations of the role of White in Malpighian tubules were carried out ~30·years ago, and tentatively identified location of, and transport substrates for, White. The location of White in the tubules was thought to be either the basolateral membrane (Sullivan et al, 1980) or the pigment storage vesicle membranes (Sullivan et al, 1979). Potential substrates for White included tryptophan (Sullivan et al, 1980), kynurenine (Sullivan and Sullivan, 1975), 3-hydroxykynurenine (Howells et al, 1977), guanine and riboflavin (Sullivan et al, 1979).…”

Overexpression of kermit/dGIPC is associated with lethality in Drosophila melanogaster

“…We observed that, rather than a chimeric protein, the aberrant mRNA (LpinK-w) expressed in the KG00562 fly line encodes an aminoterminal deleted white protein containing the ABC domain (ATP Binding Cassette; Figure 2). The ABC domain is required for the white function in transporting molecules across membranes, such as the pigment precursors tryptophan and guanine (22)(23)(24), and the cGMP in Malpighian tubules (25). Lack of the white function impairs pigment production in the light-screening cells of the compound eye resulting in flies with white eyes, which is the most prominent phenotype of w mutants.…”

Screening ABCG1, the human homologue of the Drosophila white gene, for polymorphisms and association with bipolar affective disorder

“…1 The Drosophila white gene is also involved in cellular uptake of tryptophan in Malphigian (proximal) tubule cells, where tryptophan derived from white-dependent transport is believed to enter intracellular pools of tryptophan that are distinct from those used for protein synthesis. 2 The high homology of ABCG1 to the Drosophila white gene suggests that it may also encode a tryptophan transporter, although the function of the mammalian gene remains unproven. Tryptophan transport into sero-tonergic neurones may be an important regulatory step for serotonergic neurotransmission.…”