Proteolytic cleavage of the developmentally important cadherin BT-R1 in the midgut epithelium of Manduca sexta

BT-R-1 (M-r = 210 kDa) represents a new type of insect cadherin that is expressed specifically in the midgut epithelium during growth and development of Manduca sexta larvae. It also is a target receptor for the Cry1A toxins of the entomopathogenic bacterium Bacillus thuringiensis. Expression of BT-R-1, which varies during larval development, correlates with the abundance of the protein and with the differential cleavage of the molecule at each developmental stage. The cleavage of BT-R-1, is calcium dependent, and consequently, Ca2+ directly influences the structural integrity of BT-R-1. Indeed, removal of calcium ions by chelating agents promotes cleavage of the BT-R-1 ectodomain, resulting in formation of fragments that are similar to those observed during larval development. Partial purification of proteins from brush border membrane vesicles (BBMVs) by gel filtration chromatography hinders the cleavage of BT-R-1 in the presence of EDTA and EGTA, indicating that there is specific proteolytic activity associated with the BBMV. This specific proteolytic cleavage of BT-R-1 not only alters the integrity of BT-R-1 but it most likely is implicated in cell adhesion events during differentiation and development of M. sexta midgut epithelium. We propose a model for calcium-dependent protection of BT-R-1 as well as a cleavage pattern that may modulate the molecular interactions and adhesive properties of its ectodomain. Molecular characterization of such a protection mechanism should lead to a better understanding of how the function of specific cadherins is modulated during tissue differentiation and insect development.