The lppC gene of Streptococcus equisimilis encodes a lipoprotein that is homologous to the e (P4) outer membrane protein from Haemophilus influenzae.
We report the cloning, sequencing, and analysis of a novel chromosomal gene of Streptococcus equisimilis strain H46A that codes for a membrane lipoprotein, designated LppC. The lppC gene is located 3' adjacent to, and co-oriented with, the unrelated gapC gene that encodes the previously characterized glyceraldehyde-3-phosphate dehydrogenase. Sequencing of lppC revealed an 855-bp open reading frame that predicted a 32.4-kDa polypeptide possessing a potential lipoprotein signal sequence and modification site (VTGC). Signal sequence processing of LppC synthesized in the homologous host or expressed from plasmid pLPP2 in Escherichia coli was sensitive to globomycin, a selective inhibitor of lipoprotein-specific signal peptidase II. Subcellular localization of LppC using polyclonal antibodies raised to the hexahistidyl-tagged protein proved LppC to be tightly associated with the cytoplasmic membrane of S. equisimilis and with the outer membrane of E. coli JM109 (pLPP2). Southern, Northern and Western analyses indicated that lpp was conserved in S. pyogenes, and transcribed independently of gap as monocistronic 0.9-kb mRNA from a sigma 70-like consensus promoter. Database searches found homology of LppC to the hel gene-encoded outer membrane protein e (P4) from Haemophilus influenzae to which it exhibits 58% sequence similarity. However, unlike the hel gene, lppC was unable to complement hemA mutants of E. coli for growth on hemin as sole porphyrin source in aerobic conditions. Furthermore, neither the wild type nor an lppC insertion mutant of S. equisimilis could grow on hemin in iron-limited medium. These results, together with findings indicating that S. equisimilis H46A had no absolute requirement for iron, led us to conclude that lppC, in contrast to hel, is not involved in hemin utilization and has yet to be assigned a function.