ABSTRACT
The thi n filament s consis t o f basicall y thre e proteins : actin , tropomyosin ,
and troponin . Act i n ha s a molecula r weigh t o f abou t 43,00 0 an d i s globula r ( G-
actin). I t accounts fo r abou t 25 % o f al l muscle protein . I n the presenc e o f M g 2 +
and ATP , acti n polymerize s t o for m a beaded strin g o f acti n monomer s calle d F-
actin (fibrou s actin) . Tw o suc h beade d string s interac t t o for m acti n doubl e
helices. Associate d wi t h th e F-acti n polymer s ar e th e tropomyosi n a n d troponi n
molecules. Th e forme r i s a fibrou s protein , molecula r weigh t abou t 66,000 , wi t h
two subunit s o f identica l molecula r weigh t an d bot h i n th e a-helica l con -
formation formin g a coile d coil . I t i s situate d i n th e F-acti n groove , an d i n th e
relaxed stat e i t inhibit s th e act in-myos i n interaction . Troponi n i s a globula r
protein, molecula r weight abou t 76,000 , wi t h thre e subunits : TnT , T n l, and T n C .
T n T bind s t o tropomysin , wherea s T n C bind s C a 2 + . T n l also inhibit s th e interac -
tion betwee n acti n an d myosin , a norma l situatio n i n th e restin g muscle . Th e
interrelationships amon g th e thre e protein s t o for m a thin filament ar e illustrated
