Identification and functional characterization of a peptidoglycan recognition protein from the cotton bollworm,...

Peptidoglycan recognition proteins (PGRPs) specifically bind to peptidoglycans, and play crucial roles as pattern recognition receptors (PRRs) in mediating innate immune responses. In this study, we identified and characterized a PGRP (HaPGRP-D) from the cotton bollworm, Helicoverpa armigera. Sequence analysis indicated that HaPGRP-D is an amidase-type PGRP. Expression of HaPGRP-D was upregulated in the hemocytes of H. armigera larvae after injecting Gram-negative Escherichia coli, Gram-positive Staphylococcus aureus, or chromatography beads. To test the biological activity of HaPGRP-D, purified recombinant protein was prepared. Subsequent analysis showed that rHaPGRP-D (i) could bind and agglutinate Gram-negative E. coli and Gram-positive S. aureus in a zinc-dependent manner, (ii) functioned as an amidase to degrade peptidoglycans in the presence of Zn(2+) , (iii) strongly inhibited the growth of E. coli and S. aureus in the presence of Zn(2+) , (iv) could bind to the surface of hemocytes, (v) increased the phagocytosis of E. coli cells by hemocytes in vitro, and (vi) promoted hemocyte encapsulation on chromatography beads in vitro. These results suggest that HaPGRP-D plays important roles as PRR, amidase, and opsonin in H. armigera humoral and cellular immune responses.