Abstract
A large number of tropomyosin (Tm) isoforms function as gatekeepers of the actin filament, controlling the spatiotemporal access of actin-binding proteins to specialized actin networks. Residues ∼40-80 vary significantly among Tm isoforms, but the impact of sequence variation on Tm structure and interactions with actin is poorly understood, because structural studies have focused on skeletal muscle Tmα. We describe structures of N-terminal fragments of smooth muscle Tmα and Tmβ (sm-Tmα and sm-Tmβ). The 2.0-Å structure of sm-Tmα81 (81-aa) resembles that of skeletal Tmα, displaying a similar super-helical twist matching the contours of the actin filament. The 1.8-Å structure of sm-Tmα98 (98-aa) unexpectedly reveals an antiparallel coiled coil, with the two chains staggered by only 4 amino acids and displaying hydrophobic core interactions similar to those of the parallel dimer. In contrast, the 2.5-Å structure of sm-Tmβ98, containing Gly-Ala-Ser at the N terminus to mimic acetylation, reveals a parallel coiled coil. None of the structures contains coiled-coil stabilizing elements, favoring the formation of head-to-tail overlap complexes in four of five crystallographically independent parallel dimers. These complexes show similarly arranged 4-helix bundles stabilized by hydrophobic interactions, but the extent of the overlap varies between sm-Tmβ98 and sm-Tmα81 from 2 to 3 helical turns. The formation of overlap complexes thus appears to be an intrinsic property of the Tm coiled coil, with the specific nature of hydrophobic contacts determining the extent of the overlap. Overall, the results suggest that sequence variation among Tm isoforms has a limited effect on actin binding but could determine its gatekeeper function.
Highlights
The role of sequence variation among Tm isoforms remains unexplored
The results suggest that sequence variation among Tm isoforms has a limited effect on actin binding but could determine its gatekeeper function
In an attempt to address some of these questions, we present here the crystal structures of three N-terminal fragments of smooth muscle Tm isoforms ␣ and , revealing the conformation of one of the most variable regions of the Tm sequence
Summary
The role of sequence variation among Tm isoforms remains unexplored. Results: We describe the structures of smooth muscle Tm isoforms. None of the structures contains coiled-coil stabilizing elements, favoring the formation of head-to-tail overlap complexes in four of five crystallographically independent parallel dimers These complexes show arranged 4-helix bundles stabilized by hydrophobic interactions, but the extent of the overlap varies between sm-Tm98 and sm-Tm␣81 from 2 to 3 helical turns. It is thought that different Tm isoforms exert specialized “gatekeeper” functions, regulating the spatiotemporal interactions of actin-binding proteins (ABPs) with discrete actin filament networks in cells [3,4,5,6,7,8,9,10,11,12,13,14] At least two such actin networks can be distinguished in smooth muscle cells, the actomyosin contractile network and the cytoplasmic (or non-muscle) network. Long Tm isoforms, including skeletal and smooth muscle ␣ and  isoforms, contain 284 amino acids, organized into seven pseudo-repeats of ϳ40 aa, with the rise per pseudo-repeat corresponding to
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
