Abstract
Smooth muscle myosin filaments are much less stable than the skeletal muscle counterpart. Smooth myosin requires higher concentration of Mg2+ than skeletal myosin to form thick filaments and addition of ATP disassembles the dephosphorylated smooth muscle myosin filaments into monomers but not phosphorylated ones. We found that the addition of caldesmon to dephosphorylated myosin induced the formation of the filaments under the conditions where myosin by itself is soluble or disassembled. Although the induced filaments were short at 1 mM Mg2+, they became medium sized and seemed like side polar filaments with prominent 14 nm periodicity at higher Mg2+ conditions (8 mM). In the presence of F-actin, myosin filaments induced by caldesmon were associated along actin filaments to form large structures. The association of actin and myosin filaments was observed only in the presence of caldesmon, suggesting that caldesmon cross-linked actin and myosin filaments. This cross-linking was disrupted by the addition of calmodulin. Caldesmon-induced filament formation of dephosphorylated myosin in the presence of Mg(2+)-ATP may explain the existence of myosin filaments in relaxed smooth muscle fibers. A similar effect of telokin on myosin filament assembly was also examined and is discussed.
Highlights
AR388 88, AR41653, and HL37117 and partly by grant-in-aid for Scientific Research from the Ministry of Education, Science and Culture of Japan
Which can bind to calmodulin and actin raises the basic tone of skinned smooth muscle cells
Three actin binding regions were mapped to the COOHterminal domain (Mornet et al, 1988 ; Wang et al, 1991), and the calmodulin binding region lies between residues 659-665 (Bartegi et al, 1990; Wang et al ., 1991; Hayashi et al, 1991)
Summary
(Received for publication, February 3, 1994, and in revised form, November 14, 1994). Phosphorylated myosin filaments are more stable and are not disassembled in the presence of Mg2+ATP (Suzuki et al, 1978) This may suggest that in smooth muscle, myosin filament formation may be regulated by. 15 mg/ml ca ldesmon ) in bu ffer (50 rnxt KCl, 1 or 8 mxt MgCI", 0.5 mxi ATP, 1 mxt EGTA , 5 mxt sodium ph osph ate, pH 7.5 ) we re a pplied to u ncoat ed n u mber 400 copper grids followed by 1% uran yl acet a te conta ini ng bacit racin as descr ibed prev iously (Ka taya ma , 1989b ) When t he sa mple wa s too t hi ck , it wa s dil uted five times wit h t he sam e buffe r prior to t he nega t ive sta in ing. T he sta ti st ics on th e dist ri but ion of t he filam en t lengt h was don e with the sa me eq uipme nt
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