Identification of functional differences between recombinant human ? and ? cardiac myosin motors

The myosin isoform composition of the heart is

dynamic in health and disease and has been shown to affect

contractile velocity and force generation. While different

mammalian species express different proportions of a and

b myosin heavy chain, healthy human heart ventricles

express these isoforms in a ratio of about 1:9 (a:b) while

failing human ventricles express no detectable a-myosin.

We report here fast-kinetic analysis of recombinant human

a and b myosin heavy chain motor domains. This represents the ?rst such analysis of any human muscle myosin

motor and the ?rst of a-myosin from any species. Our

?ndings reveal substantial isoform differences in individual

kinetic parameters, overall contractile character, and predicted cycle times. For these parameters, a-subfragment 1

(S1) is far more similar to adult fast skeletal muscle myosin

isoforms than to the slow b isoform despite 91% sequence

identity between the motor domains of a- and b-myosin.

Among the features that differentiate a- from b-S1: the

ATP hydrolysis step of a-S1 is *ten-fold faster than b-S1,

a-S1 exhibits *?ve-fold weaker actin af?nity than b-S1,

and actin a-S1 exhibits rapid ADP release, which is > tenfold faster than ADP release for b-S1. Overall, the cycle

times are ten-fold faster for a-S1 but the portion of time

each myosin spends tightly bound to actin (the duty ratio)

is similar. Sequence analysis points to regions that might

underlie the basis for this ?nding.