SERINE PEPTIDASE HOMOLOG FROM THE BEETLE Tenebrio molitor WITH SUBSTITUTION OF SERINE RESIDUE WITH THREONINE IN THE CATALYTIC TRIAD

Analysis of the genomes and transcriptomes of the beetle Tenebrio molitor revealed a group of six serine peptidase homologs (SPH) of the S1A chymotrypsin subfamily containing a conservative substitution of the catalytic residue Ser195 with Thr (Ser195Thr) in the active center. All six SPH are secreted proteins with prepropeptides and lack regulatory domains in the propeptide. The most highly expressed homolog, SerPH122, shares 57% sequence identity with the most highly expressed elastase-like peptidase of T. molitor, SerP41. Both proteins exhibit similar domain organization, localization in the posterior midgut, and expression patterns in the feeding stages of the fourth instar larva and imago. Testing hydrolytic activity of the recombinant rSerPH122 preparation demonstrated that the conservative substitution of Ser for Thr in the active center did not abolish its catalytic activity. rSerPH122 exhibits low specific activity but broad substrate specificity, most effectively hydrolyzing substrates of chymotrypsin-like and trypsin-like peptidases. The homolog has a pH optimum at 8.5 and is stable in the pH range 4.0-8.0. This study addresses the question of activity of the homologs with the Ser195Thr substitution and contributes to understanding of the poorly studied area of SPH functions, providing a basis for elucidating relationship between the structure and function of serine peptidases and their homologs.