Beyond the primary structure of Kazal domains in decapod crustaceans

Abstract

Kazal proteinase inhibitors (KPIs) have been described as important regulators of serine proteinases in invertebrate physiological processes, including the immune response. Several Kazal proteins have been identified in the hemocytes of crustaceans, including freshwater crayfishes, penaeid shrimps, and other shrimps and crabs. Kazal proteins have a repetitive homologous domain structure, and this molecular architecture complicates phylogenetic comparison. Such comparison, with a proper delineation of repeats at the sequence level, however, is important for understanding the structure and function of proteins as well as for the detection of homologous sequences that contribute to explain their evolutionary past. The duplication of coding sequences is a mechanism known to facilitate evolution, and the most important divergence of KPIs appears to be due to insertion, deletion or re-arrangement of complete Kazal domains. A comparison of individual domains, instead of the full protein sequence, thus seems to provide more information and allows a description of the molecular architecture and domain distribution. Considering that each domain undergoes mutation independently, variability in primary sequence via nucleotide substitution is more frequent than the tolerable insertion or deletion of a codon (one amino acid). In addition to primary structure, the distances between Cys residues could therefore be used for domain comparison. Kazal domains from decapod crustaceans are compared using their inter-Cys distances, a nomenclature is proposed for future studies, and the molecular architecture or domain arrangement of these proteins is presented.