Myosin-1C differentially displaces tropomyosin isoforms altering their inhibition of motility.

ABSTRACT

Force generation and motility by actomyosin in nonmuscle cells are spatially regulated by ∼40 tropomyosin (Tpm) isoforms. The means by which Tpms are targeted to specific cellular regions and the mechanisms that result in differential activity of myosin paralogs are unknown. We show that Tpm3.1 and Tpm1.7 inhibit Myosin-IC (Myo1C), with Tpm1.7 more effectively reducing the number of gliding filaments than Tpm3.1. Strikingly, cosedimentation and fluorescence microscopy assays revealed that Tpm3.1 is displaced from actin by Myo1C and not by myosin-II. In contrast, Tpm1.7 is only weakly displaced by Myo1C. Unlike other characterized myosins, Myo1C motility is inhibited by Tpm when the Tpm-actin filament is activated by myosin-II. These results point to a mechanism for the exclusion of myosin-I paralogs from cellular Tpm-decorated actin filaments that are activated by other myosins. Additionally, our results suggest a potential mechanism for myosin-induced Tpm sorting in cells.