E1’s ion-binding sites are exposed to the cytoplasm, and it has high affinity for Na+ but low affinity for K+

TLS Online TPP Program

#Id: 8408

#Unit 2. Cellular Organization

Tropomyosin (TM) is a ropelike molecule, about 40 nm in length, binds to 7 actin subunits in an actin filament.

TLS Online TPP Program

#Id: 8409

#Unit 2. Cellular Organization

Troponin, a complex of three polypeptides, 

TN-T (tropomyosin-binding)

TN-I  (inhibitory)

TN-C (Ca2+-binding)

TLS Online TPP Program

#Id: 8410

#Unit 2. Cellular Organization

TN-C is the calcium-binding subunit, controls the

position of TM on the surface of an actin filament through the TN-I and TN-T subunits

TLS Online TPP Program

#Id: 8411

#Unit 2. Cellular Organization

In the absence of Ca2+, muscle is relaxed

TM blocks myosin’s interaction with F-actin

TLS Online TPP Program

#Id: 8412

#Unit 2. Cellular Organization

Binding of Ca2+ ions to TN-C

triggers movement of TM to a new position on the filament, thereby exposing the myosin-binding sites on actin.

TLS Online TPP Program

#Id: 8413

#Unit 2. Cellular Organization

Troponin C is closely related to the ubiquitous 

Ca2+-binding protein calmodulin