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Although skeletal muscle fatigues fairly rapidly, clams have a protein called paramyosin that allows them to sustain contractions for up to a month. What might be the role of paramyosin at the molecular or cellular level, which results in this peculiarity?

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This appears to be a homework question, which is often frowned-upon without prior research. Where have you looked so far? –  hexafraction May 28 '13 at 11:26
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Without proper background knowledge I'd say that that's for keeping the shell closed. –  zeller May 28 '13 at 16:18
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Older papers initially presumed that it was part of a mechanism called "catch" in invertebrates like molluscs which is a "sustained contraction." However, paramyosin was also found in insects. There is a very detailed review on invertebrate muscle from 2008 that you should read through (or, you know, search through) points out that recent evidence has shown that paramyosin is not involved in catch; that review also has this to say on the subject (emphasis added):

Furthermore, paramyosin’s presence in almost every invertebrate muscle (including those with relatively small thick filaments) suggests that paramyosin should not be considered a ‘special’ molecule whose presence needs explanation. Paramyosin is instead an everyday constituent of invertebrate muscles, similar, for instance, to the giant sarcomere associated proteins (see Hooper and Thuma, 2005). Indeed, a more salient question might be why vertebrates do not have paramyosin.

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Following through; OP's questions seems poorly posed. Indeed, sustained contraction is nothing special. That's what muscle tone is. –  Ryan Sep 26 '13 at 21:48
    
From an anthropocentric position, though, it's a fair question, although as I point out the better question is why we can't contract consistently but many invertebrates can. –  Amory Sep 29 '13 at 23:56
    
Yeah I'm confused by your point. Paramyosin is not involved in catch. Contracting consistently isn't difficult if you think about rigor tonus. –  Ryan Sep 30 '13 at 22:57
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