Transient kinetic analysis of the 130-kDa myosin I (MYR-1 gene product) from rat liver - A myosin I designed for maintenance of tension?

The 130-kDa myosin I (MI130), product of the myr-1 gene, is one member of the mammalian class I myosins, a group of small, calmodulin-binding mechanochemical molecules of the myosin superfamily that translocate actin filaments. Roles for MI130 are unknown. Our hypothesis is that, as with all myosins, MI130 is designed for a particular function and hence possesses specific biochemical attributes. To test this hypothesis we have characterized the enzymatic properties of MI130 using steady-state and stopped-flow kinetic analyses. Our results indicate that: (i) the Mg2+-ATPase activity is activated in proportion to actin concentration in the absence of Ca2+ (ii) the ATP-induced dissociation of actin-M-130 is much slower for MI130 than has been observed far other myosins (-Ca2+, second order rate constant of ATP binding, 1.7 x 10(4) M-1 s(-1); maximal rate constant, 32 s(-1)); (iii) ADP binds to actin-MI130 with an affinity of similar to 10 mu M and competes with ATP-induced dissociation of actin-MI130; the rate constant of ADP release from actin-M-130 is 2 s(-1); (iv) the rates of the ATP-induced dissociation of actin-MI and ADP release are 2-3 times greater in the presence of CaCl2, indicating a sensitivity of motor activity to Ca2+; and (v) the affinity of MI130 for actin (15 nM) is typical of that observed for other myosins. Together, these results indicate that although MI130 shares some characteristics with other myosins, it is well adapted for maintenance of cortical tension.