Characterization of Streptococcus gordonii SecA2 as a paralogue of SecA.
The accessory Sec system of Streptococcus gordonii is essential for transport of the glycoprotein GspB to the bacterial cell surface. A key component of this dedicated transport system is SecA2. The SecA2 proteins of streptococci and staphylococci are paralogues of SecA and are presumed to have an analogous role in protein transport, but they may be specifically adapted for the transport of large, serine-rich glycoproteins. We used a combination of genetic and biochemical methods to assess whether the S. gordonii SecA2 functions similarly to SecA. Although mutational analyses demonstrated that conserved amino acids are essential for the function of SecA2, replacing such residues in one of two nucleotide binding folds had only minor effects on SecA2 function. SecA2-mediated transport is highly sensitive to azide, as is SecA-mediated transport. Comparison of the S. gordonii SecA and SecA2 proteins in vitro revealed that SecA2 can hydrolyze ATP at a rate similar to that of SecA and is comparably sensitive to azide but that the biochemical properties of these enzymes are subtly different. That is, SecA2 has a lower solubility in aqueous solutions and requires higher Mg(2+) concentrations for maximal activity. In spite of the high degree of similarity between the S. gordonii paralogues, analysis of SecA-SecA2 chimeras indicates that the domains are not readily interchangeable. This suggests that specific, unique contacts between SecA2 and other components of the accessory Sec system may preclude cross-functioning with the canonical Sec system.
San Francisco Veterans Affairs Medical Center, University of California, San Francisco, CA 94121, USA.
Barbara A
BA
Bensing
Paul M
PM
Sullam
D000251:Adenosine Triphosphatases / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D000595:Amino Acid Sequence; D001386:Azides / Q000494:pharmacology; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D001692:Biological Transport / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D008274:Magnesium / Q000494:pharmacology; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D008957:Models, Genetic; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D016133:Polymerase Chain Reaction; D000069816:SEC Translocation Channels; D000081416:SecA Proteins; D017386:Sequence Homology, Amino Acid; D054773:Streptococcus gordonii / Q000235:genetics* / Q000378:metabolism*
D001386:Azides; D001426:Bacterial Proteins; D026901:Membrane Transport Proteins; D000069816:SEC Translocation Channels; D000251:Adenosine Triphosphatases; D000081416:SecA Proteins; D008274:Magnesium