ABSTRACT
The secretion of recombinant proteins plays an important role in their economic production and purification. The secretion efficiency depends on the responsible signal peptide (SP) in combination with the target protein and the given host and cannot be predicted so far. Due to its high plasmid stability, the lack of alkaline extracellular proteases and only few contaminating extracellular host proteins, Priestia megaterium provides a promising alternative to common Bacillus species. For the development of an easy and fast cloning and screening system to identify the SP best suited to a distinct protein, a plasmid-based SP library containing all predicted 182 Sec-dependent SPs from P. megaterium was established. The splitting of the SPs into 10 groups of individual multi-SP plasmids (pMSPs) allows their grouped amplification and application in screening approaches. The functionality of the whole library was demonstrated by enhancing the amount of the already well-secreted α-amylase AmyE by 1.6-fold. The secretion of a novel penicillin G acylase, which remained as insoluble protein inside the cells, as its native SP is unsuitable for secretion in P. megaterium, could be enhanced even up to 29-fold. Overall, only around 170 recombinant P. megaterium clones based on 50 inserted SPs had to be screened to achieve sufficient amounts for further enzyme characterizations. Thus, this newly developed plasmid-based genetic tool applicable for P. megaterium and also other Bacillus species facilitates the identification of suitable SPs for secretion of recombinant proteins.
ABSTRACT
Penicillin G acylase (PGA) catalyzes the hydrolysis of penicillin G to 6-aminopenicillanic acid and phenylacetic acid, which provides the precursor for most semisynthetic penicillins. Most applications rely on PGAs from Gram-negative bacteria. Here we describe the first three crystal structures for PGAs from Gram-positive Bacilli and their utilization in protein engineering experiments for the manipulation of their thermostability. PGAs from Bacillus megaterium (BmPGA, Tm = 56.0 °C), Bacillus thermotolerans (BtPGA, Tm = 64.5 °C), and Bacillus sp. FJAT-27231 (FJAT-PGA, Tm = 74.3 °C) were recombinantly produced with B. megaterium, secreted, purified to apparent heterogeneity, and crystallized. Structures with resolutions of 2.20 Å (BmPGA), 2.27 Å (BtPGA), and 1.36 Å (FJAT-PGA) were obtained. They revealed high overall similarity, reflecting the high identity of up to approx. 75%. Notably, the active center displays a deletion of more than ten residues with respect to PGAs from Gram-negatives. This enlarges the substrate binding site and may indicate a different substrate spectrum. Based on the structures, ten single-chain FJAT-PGAs carrying artificial linkers were produced. However, in all cases, complete linker cleavage was observed. While thermostability remained in the wild-type range, the enzymatic activity dropped between 30 and 60%. Furthermore, four hybrid PGAs carrying subunits from two different enzymes were successfully produced. Their thermostabilities mostly lay between the values of the two mother enzymes. For one PGA increased, enzyme activity was observed. Overall, the three novel PGA structures combined with initial protein engineering experiments provide the basis for establishment of new PGA-based biotechnological processes.
