RESEARCH ARTICLE Identification of functional differences between recombinant human a and b cardiac myosin motors

Abstract 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 repre-sents the first such analysis of any human muscle myosin motor and the first of a-myosin from any species. Our findings reveal substantial isoform differences in individual kinetic parameters, overall contractile character, and pre-dicted 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 *five-fold weaker actin affinity than b-S1, and actina-S1 exhibits rapid ADP release, which is [ten-fold 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 finding