Purification and interaction analyses of two human lysosomal vitamin B12 transporters: LMBD1 and ABCD4

Mutations in human LMBRD1 and ABCD4 prevent lysosomal export of vitamin B-12 to the cytoplasm, impairing the vitamin B-12-dependent enzymes methionine synthase and methylmalonyl-CoA mutase. The gene products of LMBRD1 and ABCD4 are implicated in vitamin B-12 transport at the lysosomal membrane and are proposed to act in complex. To address the mechanism for lysosomal vitamin B-12 transport, we report the novel recombinant production of LMBD1 and ABCD4 for detailed biophysical analyses. Using blue native PAGE, chemical crosslinking, and size exclusion chromatography coupled to multi-angle light scattering (SEC-MALS), we show that both detergent-solubilized LMBD1 and detergent-solubilized ABCD4 form homodimers. To examine the functional binding properties of these proteins, label-free surface plasmon resonance (SPR) provides direct in vitro evidence that: (i) LMBD1 and ABCD4 interact with low nanomolar affinity; and (ii) the cytoplasmic vitamin B-12-processing protein MMACHC also interacts with LMBD1 and ABCD4 with low nanomolar affinity. Accordingly, we propose a model whereby membrane-bound LMBD1 and ABCD4 facilitate the vectorial delivery of lysosomal vitamin B-12 to cytoplasmic MMACHC, thus preventing cofactor dilution to the cytoplasmic milieu and protecting against inactivating side reactions.