How does Myosin I differ from Myosin II?
Myosin superfamily diversified into eighteen classes sharing conserved motor domain but divergent tail architectures dictating cargo and filament forming ability defining cellular specialization. Myosin II forms bipolar thick filaments through antiparallel association of long coiled coil tails each filament containing hundreds of molecules with heads at both ends ideal for sliding antiparallel actin during sarcomere contraction and cytokinetic ring constriction where force needed. In contrast myosin I is monomeric single headed myosin with short tail lacking coiled coil filament forming propensity. Instead its TH1 domain enriched in basic residues binds directly to acidic phosphoinositide containing membranes allowing crosslinking actin to membrane generating tension. Tail homology domains also bind adaptors for vesicle transport during endocytosis and exocytosis tensioning microvilli and stereocilia adaptation in auditory hair cells. Thus myosin I functions as membrane actin tether and vesicle transporter whereas myosin II acts as contractile filament builder illustrating how tail evolution switches mechanical output from transport to contraction.
Ref: Alberts et al., Molecular Biology of the Cell, Chapter 16 – Myosin I single headed transporter vs Myosin II bipolar filaments.