The heavy chain of Acanthamoeba myosin IB is a fusion of myosin-like and non-myosin-like sequences

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Acanthamoeba castellanii myosins IA and IB exhibit myosin catalytic activity and support contractile functions in vitro.
• However, their low molecular weight, globular shape, and inability to form filaments classify them as structurally atypical myosins.

Purpose of the Study:

• To determine the complete amino acid sequence of the 128-kDa myosin IB heavy chain.
• To elucidate the structural composition and evolutionary origins of this atypical myosin.

Main Methods:

• Deduction of the amino acid sequence from the nucleotide sequence of the myosin IB gene.
• Sequence similarity comparisons with conventional myosins and analysis of structural characteristics.

Main Results:

• The myosin IB heavy chain is a fusion protein: the N-terminal ~76 kDa is similar to conventional myosin heads.
• The C-terminal ~51 kDa shows no similarity to conventional myosin tails, is rich in glycine, proline, and alanine, and lacks alpha-helical coiled-coil structure.
• Striking conservation of intron/exon structure was observed between the myosin IB gene and a vertebrate muscle myosin gene.

Conclusions:

• Acanthamoeba castellanii myosin IB possesses a unique structure with a myosin head fused to an unusual carboxyl-terminal domain, not a typical coiled-coil rod.
• This supports the conclusion that filamentous myosin is not required for force generation.
• The conserved intron/exon structure suggests ancient origins for myosin gene organization.