The cathepsin L-like proteinases from the midgut of Tenebrio molitor larvae: Sequence, properties, immunocytochemical localization and function

CDNAs coding for five procathepsin L-like proteinases (pCALs) were cloned and sequenced from a cDNA library prepared from Tenebrio molitor larval midguts: pCAL1a (with the isoforms pCAL1b and pCAL1c), pCAL2, and pCAL3. All the pCALs have the active residues Cys 25, His 169, Asn 175, and Gln 19 (papain numbering), the ERFNIN motif of papain-like enzymes and their sequences are homologous to cathepsin L enzymes. pCAL1a was expressed in bacterial systems. It is auto-catalytically activated at low pH, has kinetic properties and N-terminal sequence identical to hemocyte cathepsin L-like proteinase (CAL) and was used to raise antibodies. Semi-quantitative RT-PCR data showed that mRNAs for pCAL2 and pCAL3 were transcribed in midgut and in lesser amounts in hemolymph, whereas that for pCAL1a was transcribed in these tissues and also in fat body, Malpighian tubules, and carcass. Imunochemical detection recognized pCAL1a translation in all tissue homogenates, except anterior midgut. At this region, the presence of pCAL2 is suggested on the grounds of electrophoretical migration and high recovery of CAL2 activity from anterior midgut cells and from isolated midgut contents. Immunocytochemical localization data revealed that pCAL1a occurs in lysosome-like vesicles in all tissues, except anterior midgut, where a labelling considered to correspond to pCAL2 is found in large acidic granules being released by apocrine secretion. Putative pCAL2 was also detected in midgut contents, probably in the form of CAL2, the major luminal CAL, which was purified to homogeneity. A cladogram of insect CALs result in a monophyletic branch with lysosomal T. molitor enzymes and enzymes from five insect orders and in a polyphyletic array of coleopteran sequences, including digestive CALs from T. molitor. The data suggest that only Coleoptera have digestive CALs that may originate by gene duplication and independent evolution relative to the gene encoding the lysosomal enzyme.