Efficient secretory production of proline/alanine/serine (PAS) biopolymers in Corynebacterium glutamicum yielding a monodisperse biological alternative to polyethylene glycol (PEG)

BACKGROUND: PAS biopolymers are recombinant polypeptides comprising the small uncharged l-amino acids Pro, Ala and/or Ser which resemble the widely used poly-ethylene glycol (PEG) in terms of pronounced hydrophilicity. Likewise, their random chain behaviour in physiological solution results in a strongly expanded hydrodynamic volume. Thus, apart from their use as fusion partner for biopharmaceuticals to achieve prolonged half-life in vivo, PAS biopolymers appear attractive as substitute for PEG—or other poorly degradable chemical polymers—in many areas. As a prerequisite for the wide application of PAS biopolymers at affordable cost, we have established their highly efficient biotechnological production in Corynebacterium glutamicum serving as a well characterized bacterial host organism. RESULTS: Using the CspA signal sequence, we have secreted two representative PAS biopolymers as polypeptides with ~ 600 and ~ 1200 amino acid residues, respectively. Both PAS biopolymers were purified from the culture supernatant by means of a simple downstream process in a truly monodisperse state as evidenced by ESI–MS. Yields after purification were up to ≥ 4 g per liter culture, with potential for further increase by strain optimization as well as fermentation and bioprocess development. Beyond direct application as hydrocolloids or to exploit their rheological properties, such PAS biopolymers are suitable for site-specific chemical conjugation with pharmacologically active molecules via their unique terminal amino or carboxyl groups. To enable the specific activation of the carboxylate, without interference by the free amino group, we generated a blocked N-terminus for the PAS(1200) polypeptide simply by introducing an N-terminal Gln residue which, after processing of the signal peptide, was cyclised to a chemically inert pyroglutamyl group upon acid treatment. The fact that PAS biopolymers are genetically encoded offers further conjugation strategies via incorporation of amino acids with reactive side chains (e.g., Cys, Lys, Glu/Asp) at defined positions. CONCLUSIONS: Our new PAS expression platform using Corynex(®) technology opens the way to applications of PASylation(®) technology in multiple areas such as the pharmaceutical industry, cosmetics and food technology.