The 130-kDa myosin I (MI(130)), 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 MI(130) are unknown. Our hypothesis is that, as with all myosins, MI(130) is designed for a particular function and hence possesses specific biochemical attributes. To test this hypothesis we have characterized the enzymatic properties of MI(130) using steady-state and stopped-flow kinetic analyses. Our results indicate that: (i) the Mg(2+)-ATPase activity is activated in proportion to actin concentration in the absence of Ca(2+); (ii) the ATP-induced dissociation of actin-MI(130) is much slower for MI(130) than has been observed for other myosins (-Ca(2+), 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-MI(130) with an affinity of approximately 10 microM and competes with ATP-induced dissociation of actin-MI(130); the rate constant of ADP release from actin-MI(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 CaCl(2), indicating a sensitivity of motor activity to Ca(2+); and (v) the affinity of MI(130) for actin (15 nM) is typical of that observed for other myosins. Together, these results indicate that although MI(130) shares some characteristics with other myosins, it is well adapted for maintenance of cortical tension.