Overlapping distribution of the 130- and 110-kDa myosin I isoforms on rat liver membranes

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.