{"CAPTION FIG1.png": "'\\n\\n**(b)** Fit to the cryo-EM map of the W loop of the terminal subunit (blue) versus a subunit from the middle of the filament (gray). **(E)** In the middle of F-actin, the W loop and the C terminus of actin form a pinch-like structure that grasps the D loop of the subunit below (gray and cyan, respectively). This interaction is absent for subunits at the barbed end (blue), resulting in the W loop adapting a G-actin conformation and the C terminus moving by 2.0 A (red arrows). Single-letter abbreviations for amino acids referenced throughout are as follows: R. Agi, K. Lysi, L. Ile, P. Pro, F. Phe, Q. Qin, G. Gly, E. Glic, H. Hia; L. Leu: N. Asri, A. Alis; T. Thr\\n\\nFig. 1: **Free barbed end.** (**A**) Schematic representation of the G- to F-actin transition. A crosswhite -20\\\\({}^{\\\\circ}\\\\) rotation of the outer domain (subdomains 1 and 2) is definite to the inner domain (subdomains 3 and 4) produces a flatter conformation of subunits in F-actin. **(B)** Cryo-EM map of the free barbed end at 3.30-A resolution. The terminal and perultimate subunits are shown in two different shades of blue. **(C)** The terminal and penultimate subunits (blue) adopt the classical F-actin conformation of subunits in the middle of F-actin (gray). **(S)** Stimulus were superimposed based on the inner domain (surface representation) to highlight differences in orientation of the outer domain (ribbon representation).\\n\\n'", "CAPTION FIG2.png": "\"(**E**) Comparison of the filament-bound (pink, magenta) and unbound (\\\\(Z\\\\)Z) (gray, PDB code 3AA7) structures of CapZ. The superimposition, based on CapZa, highlights a -15\\\\({}^{\\\\circ}\\\\) rotation of CapZb (red arrow) that flattens the mushroom head. (**F**) The actin-binding surface of CapZ consists mostly of two antiparallel helices and the a and b tetracles. Comparison of the filament-bound (pink, magenta) and unbound (gray) structures shows that the helices contain n bulges that charge confirmation between these two states (red arrows). The tetracles, which are disordered in the unbound structure, project out in the filament-bound structure to engage the two barbed-end subunits. (**G**) Close-up view of CapZ's interaction with the barbed end, with the binding interface colored pink (CapZa) and magenta (CapZb). CapZ residues participating in the interaction are shown and labeled in fig. S9C.\\n\\nFig. 2: **CapZ-capped barbed end.** (**A**) Domain diagram of CapZ subunits \\\\(\\\\alpha\\\\) and \\\\(\\\\beta\\\\). (**B**) Macro-free structure of CapZ and transition between filament-bound and unbound states. In filament-bound CapZ, the mushroom head flattens the tetractes engage the hydrophobic elicits of barbed-end subunits. (**C**) Cry-EM trap of the CapZ-capped barbed end at 2.79-A resolution, showing the terminal and penultimate actin subunits is two states of blue and CapZa and CapZb in pink and magenta, respectively. (**D**) Solving at the short-pitch bar formed by the terminal and penultimate subunits (blue) as compared to a short-pitch pair from the middle of F-actin (gray). The short-pitch pairs were superimposed based on the penultimate subunit to highlight the playing of the terminal subunit, showing a maximum displacement of 1.5 \u00c5 resulting from a rotation of -2\\\\({}^{\\\\circ}\\\\) (red arrow). Solving is likely favored by missing contacts of the interstrand plug at the barbed end compared to the middle of F-actin (dashed red curves).\\n\\n\"", "CAPTION FIG3.png": "'Figure 3: **Free pointed end.** (**A**) Cryo-EM map of the free pointed end at 2.84-A resolution, showing the first and second subunits in two different shades of green. The D long of subunits at the pointed end is disordered. (**B**) The first (left) and second (right) subunits at the pointed end adopt a G-actin conformation, where the outer domain is rotated -20\\\\({}^{\\\\circ}\\\\) (red arrow) compared with subunits in the middle of F-actin (gray). Subunits were superimposed based on their inner domains (surface representation) to highlight differences in the orientation of their outer domains (ribeon representation).\\n\\n'", "CAPTION FIG4.png": "'by the b-strand to a-helix loops of the LRR domain of ABS2. Timod residues involved in the interaction are shown and labeled in fig. SBD. (**C**) The first actin subunit (left) adopts a G-actin conformation with the outer domain rotated by -20\\\\({}^{\\\\circ}\\\\) (red arrow) compared with subunits in the middle of F-actin (gray). The second actin subunit (right) adopts an F-actin conformation. Subunits were superimposed based on their inner domains (surface representation) to highlight differences in orientation of their outer domains (ribleon representation).\\n\\nFig. 4: **Tmod-capped pointed end.** (**A**) Domain diagram of Timod, comprising alternating tropomyosin (TMSSI and TMSS2, disordered in the structure) and fraction (ABS1 and ABS2) binding sites. (**B**) Cryo-EM map of the Tmod-capped pointed end at 3.26-A resolution, showing the first and second subunits in two shades of green and Timod in orange. The view on the right is approximately taken the longitudinal axis of F-actin and shows a ribbon representation of Timod. ABS1 caps the first actin subunit, whereas ABS2 wedges into a cleft formed by the first three actin subunits. Most of the interactions with actin are mediated by the b-strand to a-helix loops of the LRR domain of ABS2. Timod residues involved in the interaction are shown and labeled in fig. SBD. (**C**) The first actin subunit (left) adopts a G-actin conformation with the outer domain rotated by -20\\\\({}^{\\\\circ}\\\\) (red arrow) compared with subunits in the middle of F-actin (gray). The second actin subunit (right) adopts an F-actin conformation. Subunits were superimposed based on their inner domains (surface representation) to highlight differences in orientation of their outer domains (ribleon representation).\\n\\n'", "CAPTION FIG5.png": "'\\n\\n**Fig. 5. Model of subunit association and dissociation at the free and capped ends of F-actin.** Structures described here show that subunits in F-actin have different conformations depending on whether they are in the middle or at the ends of F-actin, and are different from those of G-actin. Notably, these are not nucleotide-dependent conformational differences, which are relatively minor in both G- and F-actin (see text). The conformational differences at the ends of F-actin correlate with the association and dissociation constants of subunits at the ends of F-actin. Only the main pathway at equilibrium is depicted, with ATP-actin preferentially adding to the barbed end and ADP-actin dissociating from the parifed end [see (_I_) for other possible reactions]. Structural differences explain the asymmetric association of ATP-actin monomers to the barbed and parifed ends of F-actin, with ATP-bound monomers more likely to undergo the G- to F-actin transition required for preferential binding to the barbed end than ADP-bound monomers. CapZ and Trzed inhibit subunit exchange at the barbed and pointed ends, respectively, by structural mechanisms revealed in this study.\\n\\n'"}