When a lion tears apart its quarry, it may have more in common with a nut-nibbling squirrel than you think. Peter Reiser from Ohio State University, USA, explains that when muscles contract, two muscle filaments slide past each other. One of the muscle filaments is made up of a protein called myosin, and this protein is specially adapted in the jaws of carnivores to give them their deadly bite. So imagine Reiser's surprise when this muscle protein turned up in the jaws of a hard-core vegetarian: the grey squirrel. Initially perplexed by this discovery, Reiser soon realised that instead of neatly nibbling away at nuts, grey squirrels obliterate them, in much the same way that carnivores clamp their jaws around their victims. Despite their vegetarian lifestyle, grey squirrels need as much jaw power, to shatter nuts, as a carnivore does. Having made this initial discovery, Reiser was curious to discover whether carnivores and rodent jaws have other proteins in common. Explaining that muscular contractions are regulated by two proteins, troponin and tropomyosin, both of which could modulate bite strength, Reiser decided to find out whether the rodent jaw versions of these proteins are carnivore-like or more similar to those of other rodents ().
Separating jaw and limb muscle components of 27 species – ranging from rodents and marsupials to bats, omnivores and carnivores – on electrophoresis gels, checking their identities using antibodies and measuring the proteins' masses, Reiser and his colleagues found that almost all of the rodents had the carnivore type of myosin in their jaw muscles. However, when Reiser and his co-workers analysed the expression patterns of the different forms of troponin and tropomyosin, they found that the rodents produced a different form of tropomyosin from the carnivores, and the marsupials' troponin and tropomyosin proteins were intermediate between the two.
Comparing the animals' dining habits with their troponin and tropomyosin patterns, Reiser suspects that having rodent-like troponin and tropomyosin allows rodents to nibble hard and fast while having carnivore-like proteins allows meat eaters to grip on hard for long periods. However, Reiser points out that omnivores and carnivores have similar distributions of both proteins despite their different dining habits, while the marsupials all have similar protein expression patterns despite having a wide range of diets. He is keen to find out more about the contractile performance of jaw-closing muscles to better understand the protein expression patterns that he has found.
