Abstract
The ATP binding cassette, class A (ABCA) proteins are homologous polytopic transmembrane transporters that function as lipid pumps at distinct subcellular sites in a variety of cells. Located within the N terminus of these transporters, there exists a highly conserved xLxxKN motif of unknown function. To define its role, human ABCA3 was employed as a primary model representing ABCA transporters, while mouse ABCA1 was utilized to support major findings. Transfection studies showed colocalization of both transporters with surfactant protein C (SP-C), a marker peptide for successful protein targeting to lysosomal-like organelles. In contrast, alanine mutation of xLxxKN resulted in endoplasmic reticulum retention. As proof of principle, swapping xLxxKN for the known lysosomal targeting motif of SP-C resulted in post-Golgi targeting of the SP-C chimera. However, these products failed to reach their terminal processing compartments, suggesting that the xLxxKN motif only serves as a Golgi exit signal. We propose a model whereby an N-terminal signal sequence, xLxxKN, directs ABCA transporters to a post-Golgi vesicular sorting station where additional signals may be required for selective delivery of individual transporters to final subcellular destinations.
Highlights
The ATP binding cassette, class A (ABCA) proteins are homologous polytopic transmembrane transporters that function as lipid pumps at distinct subcellular sites in a variety of cells
Using alveolar epithelial cell line A549, expression of human ABCA3 (hABCA3)/enhanced green fluorescent protein (EGFP) displayed nearly total colocalization with CD63-positive vesicles (a marker antigen associated with lamellar bodies and lysosome-like vesicles [28] (Fig. 2A, top row)
In A549 cells, only the first of four cleavages occurs in early post-Golgi compartments in a non-cell-specific manner (Fig. 1). Both hABCA3 and surfactant protein C (SP-C) are trafficked 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]
Summary
The ATP binding cassette, class A (ABCA) proteins are homologous polytopic transmembrane transporters that function as lipid pumps at distinct subcellular sites in a variety of cells. Located within the N terminus of these transporters, there exists a highly conserved xLxxKN motif of unknown function. As proof of principle, swapping xLxxKN for the known lysosomal targeting motif of SP-C resulted in post-Golgi targeting of the SP-C chimera. These products failed to reach their terminal processing compartments, suggesting that the xLxxKN motif only serves as a Golgi exit signal. We propose a model whereby an N-terminal signal sequence, xLxxKN, directs ABCA transporters to a post-Golgi vesicular sorting station where additional signals may be required for selective delivery of individual transporters to final subcellular destinations.—Beers, M. A novel conserved targeting motif found in ABCA transporters mediates trafficking to early post-Golgi compartments.
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