Secretory proteins of Chironomus salivary glands: structural motifs and assembly characteristics of a novel biopolymer

Abstract

Salivary glands of Chironomus synthesize a family of at least ten secretory proteins that can be grouped into three size classes: the large (about 1000 kDa), intermediate (100- to 200 kDa), and small (less than 100 kDa). After synthesis, secretory proteins undergo a dramatic transformation to form a novel biopolymer. Secretory proteins accumulate in the central lumen of the gland, forming dissociable complexes that appear as a network of smooth fibrils and multistranded beaded fibers. When secretory protein complexes are extruded through the secretory duct, the fibers become oriented in parallel arrays; when these parallel arrays of fibers emerge from the mouth of larvae they are an insoluble, silk-like thread. Regulation of secretory protein-coding gene expression determines which secretory proteins are synthesized, thus, the composition of silk threads. At least two types of threads are produced: larval silk is used to construct tubes for protective housing and assist with feeding; prepupal silk is used to construct tubes for larval/pupal ecdysis (pupation). Variations in composition presumably contribute to different mechanical properties of larval and prepupal silk threads. Since the macroscopic physical properties of polymerized silk most likely reflect the microscopic structure and interaction of secretory proteins, it becomes important to learn the principles which govern secretory protein assembly at the molecular level. Which secretory proteins interact and what are the sites used for intraportein and protein-protein interactions during the assembly of this biopolymer? All eight secretory proteins characterized thus far contain tandemly repeated peptide sequences (ranging from 14-90 amino acids in length) and/or a periodic distribution of Cys residues. These motifs appear to be unique; no other biopolymer has either the repeated peptide sequences or composite structure of chironomid silk threads. The evolutionary conservation of motifs within repeats and among different secretory proteins suggests that the sequences and three-dimensional structures of the motifs may be important for assembly of secretory proteins into complexes, oriented fibers, and silk threads. Further study of secretory protein assembly will bring us closer to understanding how this silk assembles in vivo. By learning principles that nature employs to construct such a novel composite biopolymer, it may become feasible to design and produce new classes of fibers or biomolecular materials with distinctive properties that are currently unavailable.