The Folded-to-Extended Conformational Transition

A striking feature of smooth muscle myosin is that upon addition of stoichiometric amounts of MgATP, 

FIG U R E 2 Metal-shadowed smooth muscle myosin in the folded and extended conformation. Upper panel At physiological ionic strength in the presence of MgATP, myosin preferentially forms a folded monomer where the tail is bent into approximately equal thirds. Note the heads often bend down toward the rod. Lower panel For comparison, extended monomers formed at high ionic strength are shown. Bar-50 nm. Reprinted from Trybus, K. M., and Lowey, S., Journal of Biological Chemistry 259, 8564-8571, 1984. 

1978 Trybus et al., 1982 Trybus and Lowey, 1984 Craig etal., 1983). Centrifugation separates the soluble folded monomer from filaments, whereas gel filtration can be used to separate folded from extended monomers because of their difference in Stokes radius (Trybus and Lowey, 1984, 1988). 

The RLC is required to form the folded monomer (Trybus and Lowey, 1988). This observation is consistent with electron microscopy images showing that the tail in the folded monomer is in close proximity to the head/rod junction where the RLC is located (Trybus and Lowey, 1984 Craig et al., 1983 Onishi and Wakabayashi, 1982). The heads in the folded monomer appear to be constrained in a downward orientation against the subragment-2 region of the rod, unlike the numerous orientations of the head relative to the rod observed in the extended monomer. 

Light Chain Mutants Identify Regions That Stabilize the Folded Monomeric Conformation 

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Mutant RLCs incorporated into gizzard myosin show that two basic residues in the N-terminal region of the RLC are necessary to stabilize the folded monomer (Table I). The folded-to-extended conformational transition is salt dependent (Trybus and Lowey, 1984), and thus ionic interactions are likely to be involved in the stabilization of the bent monomer. Eight of the first 16 residues in the N-terminal region of the RLC are... 

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