9.A.54 The Lysosomal Cobalamin (B12) Transporter (L-B12T) Family
Vitamin B12 (cobalamin) is essential in animals for metabolism of branched chain amino acids and odd chain fatty acids, and for remethylation of homocysteine to methionine. The putative lysosomal cobalamin transporter of animals (and possibly other eukaryotes), in humans, when defective, gives rise to cobalamin F-type disease (Ruivo et al., 2009; Rutsch et al., 2010). The protein has 9 TMSs as do many of its homologues. Transmembrane proteins showing sequence similarity are present in most types of eukaryotes as well as bacteria. Some of the latter resemble ABC-type transporters (e.g., 3.A.1.128.2) and RodA of E. coli (9.B.3.1.2).
In the cblF inborn error of vitamin B12 metabolism, free vitamin accumulates in lysosomes, thus hindering its conversion to cofactors. Using homozygosity mapping in 12 unrelated cblF individuals and microcell-mediated chromosome transfer, Rutsch et al., 2009 identified a candidate gene on chromosome 6q13, LMBRD1, encoding LMBD1, a lysosomal membrane protein with homology to the lipocalin membrane receptor LIMR. They identified five different frameshift mutations in LMBRD1 resulting in loss of LMBD1 function, with 18 of the 24 disease chromosomes carrying the same mutation embedded in a common 1.34-Mb haplotype. Transfection of fibroblasts of individuals with cblF with wild-type LMBD1 rescued cobalamin coenzyme synthesis and function. Thus, LMBRD1 is the gene underlying the cblF defect of cobalamin metabolism and suggests that LMBD1 is a lysosomal membrane exporter for cobalamin.
The reaction proposed to be catalyzed is:
Cobalamin (out) → Cobalamin (in)
Probable lysosomal cobalamin exporter, NESI or C61F disease protein; defective in cobalamin F type disease (540aas; 9 TMSs in a 5 + 4 arrangement) (Ruivo et al., 2009; Rutsch et al., 2009; Gailus et al., 2010; Wilcken, 2012) (also called LMBR1 domain-containing protein; liver regeneration p53-related protein, Lipocalin-interacting membrane receptor, limb region 1 protein, etc.) Homologues are present in all eukaryotes and probably some bacteria (e.g., EcsB protein of Bacillus cereus (gi#47570159; 10 TMSs; distantly related to 3.A.1.128.2). RodA of E. coli (9.B.3.1.2) may also be distantly related.
NESI of Homo sapiens (Q9NUN5)
Probable lysosomal cobalamin transporter of 646 aas and 7 TMSs in a 5 + 2 + hydrophilic domain arrangement.
CHGG_01714 of Chaetomium globosum
Probable lysosomal cobalamine exporter of 590 aas and 9 putative TMSs.
Cobalamine exporter of Ajellomyces capsulatus (Darling's disease fungus) (Histoplasma capsulatum)
Conserved plant hypothetical protein (9 TMSs)
Hypothetical protein of Ricinus communis (B9SQ26)
Uncharacterized protein of 476 aas and 9 TMSs in a 5 + 4 arrangement.
UP of Trypanosoma cruzi
Hypothetical alveolata protein conserved in Plasmodium species, of 585 aas and 9 TMSs in a 5 + 4 arrangement).
Hypothetical protein of Plasmodium knowlesi (B3L7M4)
LMBR1-like conserved protein of 612 aas and 9 TMSs in a 5 + 4 TMS arrangement.
LMBR1-like protein of Tetrahymena thermophila (Q22WA5)
Uncharacterized protein of 609 aas and 9 TMSs in a 5 + 4 TMS arrangement.
UP of Torulaspora delbrueckii (Candida colliculosa)
Uncharacterized protein of 741 aas and 9 or 10 TMSs in a 5 + 4 or 5 TMS.
UP of Cyanidioschyzon merolae
LMBR1 protein homologue of 537 aas
LMBR1 homologue of Ostreococcus tauri
Uncharacterized protein of 635 aas and 9 or 10 TMSs in a 5 (or 6) + hydrophilic domain + 4 TMSs + another hydrophilic domain. The protein shows very little sequence similarity with other members of this family.
UP of Methanosphaera stadtmanae