22 ホモポリアミノ酸ε-Poly-L-lysineを合成する新奇非リボソームペプチド合成酵素(口頭発表の部)

抄録

Only two different amino-acid "homopolymers" comprised of a single type of amino acid are known in nature: γ-poly-glutamic acid (γ-PGA) and ε-poly-L-lysine (ε-PL). ε-PL consisting of 25-35 L-lysine residues in isopeptide linkages has shown promise in medical and industrial applications. Elucidating the biosynthetic mechanism of ε-PL should open new avenues for creating novel classes of biopolymers. Here we report on the purification of an ε-PL synthase (Pls; 130kDa) and the cloning of its gene from an ε-PL-producing strain of Streptowyces albulus. Pls was found to be a membrane protein with six transmembrane domains. It was shown to be a single-module nonribosomal peptide synthase (NRPS) with traditional adenylation (A-domain) and thiolation domains (T-domain) in its amino (N)-terminal region. It had no traditional condensation or thioesterase domain; instead, it had three tandem carboxy (C)-terminal acyltransferase domains that catalysed L-lysine polymerization iteratively, yielding chains of diverse length (3-mer to 17-mer) in vitro. Furthermore, these domains catalysed the condensation of free L-lysine monomers (or ε-PL oligomers) as acceptors with enzyme-bound L-lysine as a donor; hence, L-lysine activated by the A-domain and T-domain was used only as an extender unit. A database search showed that Pls homologues were widely distributed among microorganisms and that amino-acid homopolymers, in addition to ε-PL and γ-PGA, might occur in nature.