Molecular dissection of Caenorhabditis elegans ATP-binding cassette transporter protein HAF-4 to investigate its subcellular localization and dimerization

Caenorhabditis elegans HAF-4 and HAF-9 are half-type ATP-binding cassette (ABC) transporter proteins, which are highly homologous to the human peptide transporter protein, transporter associated with antigen processing-like (TAPL, ABCB9). TAPL forms homodimers and localizes to lysosomes, whereas HAF-4 and HAF-9 form heterodimers and localize to intestine-specific non-acidified organelles. Both TAPL and HAF-4/HAF-9 are predicted to have four amino-terminal transmembrane helices [transmembrane domain 0 (TMD0)] additional to the six transmembrane helices that form the canonical core domain of ABC transporters with a cytosolic ABC region. TAPL requires its amino-terminal domain for localization to lysosomes; however, molecular mechanisms underlying HAF-4 and HAF-9 localization to their target organelles had not been elucidated. Here, we demonstrate that the mechanisms underlying HAF-4 localization differ from those underlying TAPL localization. Using transgenic C. elegans expressing mutant HAF-4 proteins labeled with green fluorescent protein, we reveal that the TMD0 of HAF-4 was not sufficient for proper localization of the protein. The mutant HAF-4, which lacked TMD0, localized to intracellular organelles similarly to the wild-type protein and functioned normally in the biogenesis of its localizing organelles, indicating that the TMD0 of HAF-4 is dispensable for both its localization and function.