The biochemical and mechanochemical properties and localization of myosin I
suggest the involvement of these small members of the myosin superfamily i
n some aspects of intracellular motility in higher cells. We have determine
d by quantitative immunoblotting with isoform-specific antibodies that the
130-kDa myosin I (myr 1 gene product) and 110-kDa myosin I (myr 2 gene prod
uct) account for 0.5 and 0.4%, respectively, of total rat liver protein. Im
munoblot analyses reveal that the 130- and 110-kDa myosins I are found in s
everal purified subcellular fractions from rat liver; The membrane-associat
ed 130-kDa myosin I is found at the highest concentration in the plasma mem
brane (28 ng/mg plasma membrane protein) followed by the endoplasmic reticu
lum-like mitochondria-associated membrane fraction (MAM; 10 ng/mu g MAM, pr
otein), whereas the 110-kDa myosin I is found at the highest concentration
in Golgi (50 ng/mu g Golgi protein:) followed by plasma membrane (20 ng/mu
g) and MAM (7 ng/mu g). Our analyses indicate that myosin I is peripherally
associated with Golgi and MAM and its presence in these fractions is not a
consequence of myosin I bound to contaminating actin filaments. Although f
ound in relatively low concentrations in microsomes, because of the abundan
ce of microsomes, in liver most of the membrane-associated myosin I is asso
ciated with microsomes. Neither myosin I isoform is detected in purified mi
tochondria. This is the first quantitative analysis addressing the cellular
distribution of these mammalian class I myosins. (C) 1999 Academic Press.