Different localizations and cellular behaviors of leiomodin and tropomodulin in mature cardiomyocyte sarcomeres.
Leiomodin (Lmod) is a muscle-specific F-actin-nucleating protein that is related to the F-actin pointed-end-capping protein tropomodulin (Tmod). However, Lmod contains a unique ∼150-residue C-terminal extension that is required for its strong nucleating activity. Overexpression or depletion of Lmod compromises sarcomere organization, but the mechanism by which Lmod contributes to myofibril assembly is not well understood. We show that Tmod and Lmod localize through fundamentally different mechanisms to the pointed ends of two distinct subsets of actin filaments in myofibrils. Tmod localizes to two narrow bands immediately adjacent to M-lines, whereas Lmod displays dynamic localization to two broader bands, which are generally more separated from M-lines. Lmod's localization and F-actin nucleation activity are enhanced by interaction with tropomyosin. Unlike Tmod, the myofibril localization of Lmod depends on sustained muscle contraction and actin polymerization. We further show that Lmod expression correlates with the maturation of myofibrils in cultured cardiomyocytes and that it associates with sarcomeres only in differentiated myofibrils. Collectively, the data suggest that Lmod contributes to the final organization and maintenance of sarcomere architecture by promoting tropomyosin-dependent actin filament nucleation.
Skwarek-Maruszewska
Aneta
A
Program in Cell and Molecular Biology, Institute of Biotechnology, University of Helsinki, Helsinki 00790, Finland.
Boczkowska
Malgorzata
M
Zajac
Allison L
AL
Kremneva
Elena
E
Svitkina
Tatyana
T
Dominguez
Roberto
R
Lappalainen
Pekka
P
D000199:Actins / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D002454:Cell Differentiation; D002467:Cell Nucleus / Q000378:metabolism; D008840:Microfilament Proteins / Q000378:metabolism*; D009119:Muscle Contraction; D009124:Muscle Proteins / Q000378:metabolism*; D050505:Mutant Proteins / Q000378:metabolism; D032383:Myocytes, Cardiac / Q000166:cytology* / Q000378:metabolism*; D009210:Myofibrils / Q000378:metabolism; D024462:Myosin Type II / Q000378:metabolism; D011485:Protein Binding; D021381:Protein Transport; D051381:Rats; D012518:Sarcomeres / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism; D051348:Tropomodulin / Q000378:metabolism*; D014335:Tropomyosin / Q000378:metabolism; D015027:Zebrafish
D000199:Actins; C545284:Lmod2 protein, rat; D008840:Microfilament Proteins; D009124:Muscle Proteins; D050505:Mutant Proteins; D051348:Tropomodulin; D014335:Tropomyosin; D024462:Myosin Type II