Identification, genomic organization, and mRNA expression of LACTB, encoding a serine beta-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 clon ed mouse cDNA encodes a novel 551-amino-acid protein, LACTB, with a predict ed amino-terminal transmembrane domain but no signal peptide. It contains a n active site motif related to C-class P-lactamases. Homologues were detect ed in sequence data from human, rat, cow, rabbit, pig, toad, zebrafish, and Caenorhabditis elegans, but not in Saccharomyces cerevisae or Drosophila m elanogaster. The genes were mapped to human chromosome 15q22.1 and mouse ch romosome 9. Sequencing of a 14.7-kb fragment of mouse genomic DNA defined s ix exons. A virtual human cDNA and a 549-residue protein, predicted from un finished genomic sequence, showed the same intron/exon structure. Northern blot analysis showed expression of the 2.3-kb mRNA predominantly in mouse l iver and human skeletal muscle. This is the first reported vertebrate examp le of this microbial peptidase family.