3. The primary sequence of the protein resilin is depicted below. Resilin is a 'super elastic rubber' found in the cuticle or exoskeleton of the regions of insects that subjected to repeated extension and retraction. Elasticity is the ability of a protein to resume its original shape after being stretched, bent, or otherwise distorted in some way. Regions of the wings of locusts or the hindlimbs of fleas contain resilin which is thought to be responsible for the 'resiliency' of these tissues to damage that could otherwise result from jumping multiple times higher than one's body length or beating one's wings at an extremely high rate. This biological rubber has an elasticity that allows resilin to be stretched up to 300% before the tissue breaks. Exon I Exon I Exon II 325 387 605 0000000000 I 000 N N-terminal elastic repeat units Chitin-binding domain | C-terminal elastic repeat units Exons 1, 2 and 3 each comprise separate functional protein domains. Exon 1 extends out of the cell into the extracellular space and exon 3 is intracellular. Exon 2 is a chitin binding domain, chitin being the major polysaccharide that comprises the exoskeleton of insects. Its function is thus to anchor the extra- and intracellular subunits of resilin to the exoskeleton of insects. A. Exon 1, which extends outside of the cell membrane, contains 18 'elastic repeat units' which are 15-mer repeats of these amino acids: GGRPSDSYGAPGGGN. These segments are attached by short (2-5) amino acid segments of opposite polarity to the repeat unit. Additionally, at the N- and C- terminus of this exon are larger segments that also have opposite polarity to the 15-mer repeat. In other words, the blue and yellow regions in the diagram below have opposite polarity. This exon, when studied in isolation, takes an extended conformation in water. In other words, it does not take any secondary structure but retains a mostly disordered structure. Upon heating, or removal of water, individual exon 1 peptides begin to aggregate into stacks, of which a portion is shown below. Explain why the protein folds this way with and without water present. You should use your knowledge of amino acids to deduce the polarity of the 15-mer repeat and thus the polarity of the connecting regions. Consider glycine ambivalent in polarity- neither polar nor nonpolar.