The molecular elasticity of the insect flight muscle proteins projectin and kettin

Abstract

Projectin and kettin are titin-like proteins mainly responsible for the high passive stiffness of insect indirect flight muscles, which is needed to generate oscillatory work during flight. Here we report the mechanical properties of kettin and projectin by single-molecule force spectroscopy. Force-extension and force-clamp curves obtained from Lethocerus projectin and Drosophila recombinant projectin or kettin fragments revealed that fibronectin type III domains in projectin are mechanically weaker (unfolding force, F u Ϸ 50 -150 pN) than Ig-domains (Fu Ϸ 150 -250 pN). Among Ig domains in Sls͞kettin, the domains near the N terminus are less stable than those near the C terminus. Projectin domains refolded very fast [85% at 15 s ؊1 (25°C)] and even under high forces (15-30 pN). Temperature affected the unfolding forces with a Q 10 of 1.3, whereas the refolding speed had a Q 10 of 2-3, probably reflecting the cooperative nature of the folding mechanism. High bending rigidities of projectin and kettin indicated that straightening the proteins requires low forces. Our results suggest that titin-like proteins in indirect flight muscles could function according to a folding-based-spring mechanism.force spectroscopy ͉ refolding ͉ single molecule ͉ titin T he success of insects as a major animal group may be attributed in part to the evolution of asynchronous flight muscles (1). In asynchronous muscles there is asynchrony between muscle electrical and mechanical activity in that a single muscle action potential can trigger a series of contractionrelaxation cycles. In the indirect flight muscle (IFM), these oscillatory contractions are produced by delayed activation in response to stretch combined with the resonant properties of the thorax (2). For example, some insect flight muscles can operate at very high frequencies (100-1,000 Hz) (1). This rapid oscillatory contraction requires that the sarcomeres are stiff. This stiffness coupled with a stretch activation response allows insects' wings to beat hundreds of times per second. The molecular basis for this mechanism is not well understood but many proteins are emerging as contributing factors. Projectin and kettin form a mechanical link between the Z-discs and the ends of the thick filaments and are responsible for a large part of the passive elasticity of insect muscles (3-5) and may be responsible for the high relaxed stiffness as a prerequisite for the stretch activation response (6).Projectin and kettin are high-molecular-weight members of the titin protein superfamily (7) and are found in invertebrate muscles. Kettin is in the I-band, and projectin is in the A-band, except in IFM, in which a large part of the molecule is in the short I-band. Projectin is an 800-to 1,000-kDa protein consisting of long sections of repeated Ig and fibronectin type III (FnIII) domains. There is also a kinase domain near the C terminus and a region rich in proline, glutamate, valine, and lysine (PEVK) near the N terminus. Immunofluorescence data indicate that, in the IFM, projectin mol...

Direct Quantification of the Attempt Frequency Determining the Mechanical Unfolding of Ubiquitin Protein

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Genetic elevation of Sphingosine 1-phosphate suppresses dystrophic muscle phenotypes in Drosophila

“…Here, we extend the muscle analysis to the level of the sarcomere (Perkins et al, 2010;Schnorrer et al, 2010). Two titinlike proteins, Projectin and Kettin (Sallimus -FlyBase), form the connecting filaments in indirect flight muscles (IFMs) that link the Z-bands to thick filaments and function in muscle elasticity during oscillatory work in flight (Lakey et al, 1990;Moore et al, 1999;Ayme-Southgate et al, 2005;Bullard et al, 2006). Because significant degradation of titin has been reported in muscle biopsies of DMD patients (Matsumura et al, 1989) and titin mutations cause muscular dystrophies and cardiomyopathies (Hackman et al, 2002;Udd et al, 2005), we analyzed the expression pattern of a titin-like protein, Projectin, in Dys mutant myofibrils.…”