Degradation and regulation of specific PM protein by a novel maize cysteine proteinase, Mir1-CP, in Spodoptera frugiperda

A unique 33-kD cysteine proteinase (Mirl-CP) accumulates in maize plants in response to caterpillar attack. Mirl-CP causes growth reduction in fall armyworm (Spodoptera frugiperda) larvae due to impaired nutrient utilization. We examined the insect's first line of defense against toxins in the diet, the peritrophic matrix (PM), which is a membrane-like structure that surrounds the food bolus to protect the insect midgut from chemical and physical damage. Mirl-CP attacked and degraded specific proteins including Insect Intestinal Mucin (IIM), which is the major protein, in the PM in vivo and in vitro. Also, the expression level of the IIM gene was significantly down-regulated in resistant plant-fed or purified Mirl-CP-fed larvae. Fifty two PM proteins were identified using MALDI-TOF-MS. Fourteen of them were differentially expressed in the resistant plant-fed larvae PM. We also examined the ultrastructural features of PM using transmission electron microscopy. TEM did not reveal major differences between the PMs.