Compositions and methods for protecting a plant from an insect pest are provided. Nucleic acid molecules encoding insect protoxins or insect toxins modified to comprise at least one proteolytic activation site that is sensitive to an insect gut protease are provided. Cleavage of a modified insect protoxin at the proteolytic activation site by an insect gut protease produces an active insect toxin in the gut of the insect pest. Cleavage of a modified insect toxin of the invention at a proteolytic activation site results in the production of an active insect toxin in the insect gut that displays improved pesticidal activity relative to the insect toxin that lacks the proteolytic activation site. Methods of using the modified insect protoxin and modified insect toxin nucleic acid sequences and the polypeptides they encode to protect a plant from an insect pest are provided. Particular embodiments of the invention further provide modified insect protoxin and modified insect toxin compositions and formulations, expression cassettes, and transformed plants, plant cells, and seeds. Insect gut proteases and the nucleic acid molecules that encode them are also disclosed herein.