Molecular structure of the myosin molecule

Molecular structure of the smooth muscle myosin molecule 

The asymmetric S-1 fragment of myosin can be divided into a globular component containing the motor domain that is formed exclusively from 

The non-helical tailpiece present at the carboxy-terminus of smooth and non-muscle myosin rods is distinct from the short tailpiece present in 

Striated muscle myosins. In smooth and non-muscle cells, two different length isoforms of the tailpiece have been identified, that result in 200 kDa (SM2) and 204 kDa (SMI) heavy chain isoforms (Nagai et al 1989, Babij and Periasamy 1989). These iso forms are expressed to the same extent at 

Another set of smooth muscle heavy chain isoforms that have either the insertion or omission of 7 amino acids in the head domain near the ATP-binding region have been identified (Babij 1993, Kelly et al 1993, White et al 1993). The insertion appears to be present in visceral smooth muscle tissues but is absent in tonic vascular smooth muscle, and appears to confer functional differences in the kinetic properties of smooth muscle myosins from visceral smooth muscle. There is evidence from studies in the motility assay that the visceral smooth muscle isoform has higher ATPase activity and moves actin filaments faster than the vascular isoform (Kelly et al 1993, Rovner et al 1997). However, because the head domain isoforms may be associated with different tail domain isoforms in different tissues, as well as with different light chain isoforms, it has been difficult to establish causal relationships between myosin heavy chain isoforms and myosin kinetics with certainty. (Reviewed by Somlyo 1993, Murphy et al 1997). 

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