Tropomyosins were first identified in neuronal systems
in 1973. Although numerous isoforms were found and
described since then, many aspects of their function
and interactions remained unknown. Tropomyosin isoforms
show different sorting pattern in neurogenesis.
As one example, TM5NM1/2 is present in developing
axons, but it is replaced by TMBr-3 in mature neurons,
suggesting that these tropomyosin isoforms contribute
differently to the establishment of the functional features
of the neuronal actin networks. We developed a
method for the efficient purification of TMBr-3 and
TM5NM1 as recombinant proteins using bacterial
expression system and investigated their interactions
with actin. We found that both isoforms bind actin filaments,
however, the binding of TM5NM1 was much
stronger than that of TMBr-3. TMBr-3 and TM5NM1
modestly affected actin assembly kinetics, in an opposite
manner. Consistently with the higher affinity of
TM5NM1 it inhibited actin filament disassembly more
efficiently than TMBr-3. Similarly to other previously
studied tropomyosins TM5NM1 inhibited the Arp2/3
complex-mediated actin assembly. Notably, TMBr-3 did
not influence the Arp2/3 complex-mediated polymerization.
This is a unique feature of TMBr-3, since so
far it is the only known tropomyosin supporting the
activity of the Arp2/3 complex, indicating that TMBr-3
may colocalize and work simultaneously with Arp2/3
complex in neuronal cells
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