Identification, genomic organization, and mRNA expression of LACTB, encoding a serine β-lactamase-like protein with an amino-terminal transmembrane domain

Database searching with bacterial serine beta -lactamases identified mouse expressed sequence tags (ESTs) with significant similarity scores. The cloned mouse cDNA encodes a novel 551-amino-acid protein, LACTB, with a predicted amino-terminal transmembrane domain but no signal peptide. It contains an active site motif related to C-class P-lactamases. Homologues were detected in sequence data from human, rat, cow, rabbit, pig, toad, zebrafish, and Caenorhabditis elegans, but not in Saccharomyces cerevisae or Drosophila melanogaster. The genes were mapped to human chromosome 15q22.1 and mouse chromosome 9. Sequencing of a 14.7-kb fragment of mouse genomic DNA defined six exons. A virtual human cDNA and a 549-residue protein, predicted from unfinished genomic sequence, showed the same intron/exon structure. Northern blot analysis showed expression of the 2.3-kb mRNA predominantly in mouse liver and human skeletal muscle. This is the first reported vertebrate example of this microbial peptidase family.