Human Tonic and Phasic Smooth Muscle Myosin Isoforms Are Unresponsive to the Loop 1 Insert

Area of Science:

• Biochemistry
• Molecular Biology
• Muscle Physiology

Background:

• Smooth muscle myosin exists as two isoforms, SMA and SMB, differing by a 7-residue peptide (i7) in loop 1 of the active site.
• Previous studies on chicken isoforms suggested the i7 deletion in SMA allosterically affects nucleotide binding and Pi release compared to SMB.

Purpose of the Study:

• To investigate the functional and structural differences between human smooth muscle myosin isoforms SMA and SMB.
• To determine if the i7 peptide deletion influences nucleotide-sensitive tryptophan (NST) fluorescence and Pi release in human isoforms.

Main Methods:

• Comparative analysis of human smooth muscle myosin SMA and SMB isoforms.
• Assessment of nucleotide-sensitive tryptophan (NST) fluorescence.
• Evaluation of Pi release kinetics in actin-activated ATPase assays.

Main Results:

• Human SMA and SMB isoforms both lack the robust NST fluorescence increments observed in chicken SMB.
• The i7 deletion in human smooth muscle myosin does not impact Pi release kinetics.
• Independent studies on human SMA and SMB motility show no functional consequence of the i7 deletion.

Conclusions:

• Human smooth muscle myosin isoforms SMA and SMB do not exhibit the functional divergence seen in chicken isoforms regarding the i7 peptide.
• The presence or absence of the i7 peptide in human smooth muscle myosin does not affect key functional parameters like Pi release or motility.
• Smooth muscle myosin demonstrates significant species-specific structural and functional divergence, cautioning against cross-species extrapolation of disease-linked mutations.