Identification of LBM180, a Lamellar Body Limiting Membrane Protein of Alveolar Type II Cells, as the ABC Transporter Protein ABCA3

Abstract

Lamellar bodies are the specialized secretory organelles of alveolar type II (ATII) epithelial cells through which the cell packages pulmonary surfactant and regulates its secretion. Surfactant within lamellar bodies is densely packed as circular arrays of lipid membranes and appears to be the product of several trafficking and biosynthetic processes. To elucidate these processes, we reported previously on the generation of a monoclonal antibody (3C9) that recognizes a unique protein of the lamellar body membrane of 180 kDa, which we named LBM180. We report that mass spectrometry of the protein precipitated by this antibody generated a partial sequence that is identical to the ATP-binding cassette protein, ABCA3. Homology analysis of partial sequences suggests that this protein is highly conserved among species. The ABCA3 gene transcript was found in cell lines of human lung origin, in ATII cells of human, rat, and mouse, as well as different tissues of rat, but the highest expression of ABCA3 was observed in ATII cells. Expression of this transcript was at its maximum prior to birth, and hormonal induction of ABCA3 transcript was observed in human fetal lung at the same time as other surfactant protein transcripts were induced, suggesting that ABCA3 is developmentally regulated. Molecular and biochemical studies show that ABCA3 is targeted to vesicle membranes and is found in the limiting membrane of lamellar bodies. Because ABCA3 is a member of a subfamily of ABC transporters that are predominantly known to be involved in the regulation of lipid transport and membrane trafficking, we speculate that this protein may play a key role in lipid organization during the formation of lamellar bodies.

A novel conserved targeting motif found in ABCA transporters mediates trafficking to early post-Golgi compartments

“…Neither mutant transporter colocalized within SP-C-positive vesicles ( Fig. 3B, E ( 21,(31)(32)(33)(34). Transient expression of these controls produced traffi cking patterns for the ABCA isoforms that were similar to those of their wild-type counterparts, with hABCA3 predominantly ( Fig.…”

“…Previously, we reported that ABCA3 is preferentially traffi cked to lamellar bodies in lung epithelial cells, and to lysosomes and lysosomal-like organelles in A549 and HEK293 cell lines ( 5,21 ). Since the mechanisms governing proper traffi cking of the ABCA3 protein or any of the other ABCA transporters are largely undefi ned, we sought to identify domain(s) mediating their normal biosynthetic routing.…”

“…1 ). Both hABCA3 and SP-C are traffi cked to the lamellar body, a highly specialized lysosomal-like organelle of alveolar epithelial cells responsible for regulated secretion of surfactant lipids and proteins ( 21,25,30 ). Lysosomal-like organelles containing proSP-C can be used as a surrogate for traffi cking in A549 cells ( Fig.…”

“…The human full-length ABCA3 WT /EGFP was generated by amplifying three overlapping ‫ف‬ 2 kb segments of cDNA by RT-PCR as described previously ( 21 ). The plasmid was used as a template to generate DsRed fusion and mutant constructs by using either the single-round, two-primer PCR amplifi cation and subcloning strategy or the one-step PCR amplifi cation and subcloning method using the QuikChange kit described above.…”

The ABCA subfamily—gene and protein structures, functions and associated hereditary diseases

“…In rats, the highest ABCA3 transcript levels are detected 1-2 days before birth [34]. As glucocorticoids are known to induce fetal maturation, Yoshida et al [54] tested the effect of intraperitoneal injection of dexamethasone (Dex) into pregnant rats.…”