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a globulin that is present in muscle plasma and that in connection with myosin plays an important role in muscle contraction.
variant of before a vowel:
actinism.
historical examples

and if mr. robert hadn’t been actin’ so much like a poor prune he’d have quit that line right there.
torchy, private sec. sewell ford

that’s what my boys was brought up to think, ‘n they’re actin’ accordin’.
peak and prairie anna fuller

“you’re actin’ a leetle nervous your own self, man,” said tom osby, keenly.
heart’s desire emerson hough

“oi don’t loike th’ way he is actin’, frankie,” whispered barney.
frank merriwell’s bravery burt l. standish

and there wa’n’t any special reason that i could see for her actin’ that way.
shorty mccabe on the job sewell ford

you’re actin’ square by my poor gal, sir, so i’m agoin’ to act square by you.
the bishop’s secret fergus hume

“i don’t know as i was actin’ right, either, to talk as i did,” she continued abruptly.
while caroline was growing josephine daskam bacon

all the time myrtle was leanin’ against ma, just like a cat, actin’, i thought.
mitch miller edgar lee masters

the nigher we got, the harder he stared; and the man in front was actin’ similar in regards to him.
the depot master joseph c. lincoln

this is altruism, an’ we’re but actin’ out what you been preachin’.
a voice in the wilderness grace livingston hill

noun
a protein that partic-p-tes in many kinds of cell movement, including muscle contraction, during which it interacts with filaments of a second protein, myosin

actin ac·tin (āk’tĭn)
n.
one of the protein components found in muscle, existing as f-actin or g-actin, into which actomyosin can be split and which acts with myosin in muscle contraction.
actin
(āk’tĭn)
a protein found in all eukaryotic cells, forming filaments that make up a main component of the cell’s supporting matrix or cytoskeleton. actin and the protein myosin together make up the contractile units (called sarcomeres) of skeletal muscle fibers.