Abstract
Mutations in human LMBRD1 and ABCD4 prevent lysosomal export of vitamin B12 to the cytoplasm, impairing the vitamin B12-dependent enzymes methionine synthase and methylmalonyl-CoA mutase. The gene products of LMBRD1 and ABCD4 are implicated in vitamin B12 transport at the lysosomal membrane and are proposed to act in complex. To address the mechanism for lysosomal vitamin B12 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 B12-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 B12 to cytoplasmic MMACHC, thus preventing cofactor dilution to the cytoplasmic milieu and protecting against inactivating side reactions.
Supplementary material available online
Supplementary Figures 1 and 2
Supplementary Tables 1 and 2.