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Published 7 January 2002. doi:10.1083/jcb.200107131
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© The Rockefeller University Press, 0021-9525/2002/1/113 $5.00
The Journal of Cell Biology, Volume 156, Number 1, January 7, 2002 113-124

Article

The carboxyl-terminal isoforms of smooth muscle myosin heavy chain determine thick filament assembly properties



Arthur S. Rovner, Patricia M. Fagnant, Susan Lowey and Kathleen M. Trybus

Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT 05405

Address correspondence to Arthur S. Rovner, Department of Molecular Physiology and Biophysics, University of Vermont, Burlington, VT 05405. Tel.: (802) 656-8004. Fax: (802) 656-0747. E-mail: rovner{at}physiology.med.uvm.edu

The alternatively spliced SM1 and SM2 smooth muscle myosin heavy chains differ at their respective carboxyl termini by 43 versus 9 unique amino acids. To determine whether these tailpieces affect filament assembly, SM1 and SM2 myosins, the rod region of these myosin isoforms, and a rod with no tailpiece (tailless), were expressed in Sf 9 cells. Paracrystals formed from SM1 and SM2 rod fragments showed different modes of molecular packing, indicating that the tailpieces can influence filament structure. The SM2 rod was less able to assemble into stable filaments than either SM1 or the tailless rods. Expressed full-length SM1 and SM2 myosins showed solubility differences comparable to the rods, establishing the validity of the latter as a model for filament assembly. Formation of homodimers of SM1 and SM2 rods was favored over the heterodimer in cells coinfected with both viruses, compared with mixtures of the two heavy chains renatured in vitro. These results demonstrate for the first time that the smooth muscle myosin tailpieces differentially affect filament assembly, and suggest that homogeneous thick filaments containing SM1 or SM2 myosin could serve distinct functions within smooth muscle cells.

Key Words: cytoskeleton; myosin heavy chains; smooth muscle; sarcomeres; uterine contraction


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