LACTB is a filament-forming protein localized in mitochondria

Abstract

LACTB is a mammalian active-site serine protein that has evolved from a bacterial penicillin-binding protein. Penicillin-binding proteins are involved in the metabolism of peptidoglycan, the major bacterial cell wall constituent, implying that LACTB has been endowed with novel biochemical properties during eukaryote evolution. Here we demonstrate that LACTB is localized in the mitochondrial intermembrane space, where it is polymerized into stable filaments with a length extending more than a hundred nanometers. We infer that LACTB, through polymerization, promotes intramitochondrial membrane organization and micro-compartmentalization. These findings have implications for our understanding of mitochondrial evolution and function. M itochondria descend from ancient Gram-negative bacteria that, through endosymbiosis, became permanent residents of eukaryotic cells (1-3). As a consequence, mitochondria and Gram-negative bacteria share several biochemical features, including DNA organization, core metabolism, and a doublemembrane architecture. In Gram-negative bacteria, but not in mitochondria, a mesh-like layer of peptidoglycan is deposited between the outer and inner membrane, offering protection against mechanical stress. Following endosymbiosis, the peptidoglycan layer lost its structural importance, and was subsequently eliminated from the early eukaryotic cell. Although eukaryotes lack peptidoglycan, proteins deriving from the penicillin-binding protein (PBP) family (4) are found in all major eukaryotic lineages, including vertebrates (5).The bacterial PBPs constitute a large family of serine proteases that is distinguished by 3 conserved amino acid motifs that contribute to the formation of the catalytic site. The -SXXKmotif contains the catalytic serine residue, which undergoes reversible acylation through substrate binding, whereas the -[SY]X[NT]-and the - [KH][ST]G-motifs contribute to substrate docking (4). PBPs catalyze the hydrolysis or transpeptidation of the terminal D-alanyl-D-alanine moiety in peptidoglycan stem peptides. The acceptor for the transpeptidation reaction is the -amino group of lysine or diaminopimelate in an adjacent stem peptide. Through these reactions, PBPs contribute to modulate the extent of peptidoglycan cross-linking during bacterial cell division and cell wall elongation (4).In contrast, the function of PBP homologues in eukaryotic organisms remains largely unexplored. Amino acid sequence analyses show that the 3 conserved amino acid motifs required for catalytic activity are conserved in all eukaryotic PBP homologues (5), suggesting that they can function as active-site serine enzymes. Within the metazoan division, nematodes harbor the largest number of PBP homologues (5), and in Caenorhabditis elegans the PBP homologue LACT-1 may be involved in pathogen recognition (6). LACTB is the only PBP homologue of mammals, and LACTB has been identified in all mammalian genomes sequenced to date (5,7,8). Recently, a causative link between LACTB and obesity was detected through gene coexp...

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