A specific C-terminal deletion in tropomyosin results in a stronger head-to-tail interaction and increased polymerization.
Tropomyosin is a 284 residue dimeric coiled-coil protein that interacts in a head-to-tail manner to form linear filaments at low ionic strengths. Polymerization is related to tropomyosin's ability to bind actin, and both properties depend on intact N- and C-termini as well as alpha-amino acetylation of the N-terminus of the muscle protein. Nalpha-acetylation can be mimicked by an N-terminal Ala-Ser fusion in recombinant tropomyosin (ASTm) produced in Escherichia coli. Here we show that a recombinant tropomyosin fragment, corresponding to the protein's first 260 residues plus an Ala-Ser fusion [ASTm(1-260)], polymerizes to a much greater extent than the corresponding full-length recombinant protein, despite the absence of the C-terminal 24 amino acids. This polymerization is sensitive to ionic strength and is greatly reduced by the removal of the N-terminal Ala-Ser fusion [nfTm(1-260)]. CD studies show that nonpolymerizable tropomyosin fragments, which terminate at position 260 [Tm(167-260) and Tm(143-260)], as well as Tm(220-284), are able to interact with ASTm(1-142), a nonpolymerizable N-terminal fragment, and that the head-to-tail interactions observed for these fragment pairs are accompanied by a significant degree of folding of the C-terminal tropomyosin fragment. These results suggest that the new C-terminus, created by the deletion, polymerizes in a manner similar to the full-length protein. Head-to-tail binding for fragments terminating at position 260 may be explained by the presence of a greater concentration of negatively charged residues, while, at the same time, maintaining a conserved pattern of charged and hydrophobic residues found in polymerizable tropomyosins from a variety of sources.
Paulucci
Adriana A
AA
Departamento de Bioquímica, Instituto de Química, Universidade de São Paulo, São Paulo, SP, Brazil.
Katsuyama
Angela M
AM
Sousa
Aurea D
AD
Farah
Chuck S
CS
D000595:Amino Acid Sequence; D001483:Base Sequence; D001704:Biopolymers / Q000737:chemistry*; D002942:Circular Dichroism; D017931:DNA Primers; D008969:Molecular Sequence Data; D017384:Sequence Deletion; D017386:Sequence Homology, Amino Acid; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet; D014335:Tropomyosin / Q000737:chemistry / Q000378:metabolism*
D001704:Biopolymers; D017931:DNA Primers; D014335:Tropomyosin