Fig. 4. The differences in filament inter- and intra-strand interfaces between Leishmania and vertebrate actins.

a H-plug of LmActin (magenta) contains an insertion of an additional aspartate (Asp269, highlighted in green) in the core of the filament that is not present in vertebrate actin (cyan). Corresponding interaction partners at the neighboring D-loop also differ between the two actins. In LmActin, the electrostatic network, formed by Glu270, Lys39, and Asp286 to His173, is hindered by aspartate insertion. Ala272Pro and His275Pro substitutions, preceding the H-plug in LmActin, decrease the flexibility of the H-plug region in comparison with vertebrate actin. Lys263 (absent in the vertebrate actin) can form a salt bridge with Glu258 located in the pointed end helix of subdomain 4, and thus link the dynamics of the H-plug region to the pointed end of the monomer. b The pointed end face of LmActin subdomain 4 contains a single amino acid deletion leading to a shorter loop between α-helix 222–233 and β-strand 238–242. In vertebrate actin, this loop can adopt an α-helical conformation that stabilizes the upstream pointed end tip. In vertebrate actin, Leu236 is inserted into the hydrophobic pocket formed by Tyr218 and Phe255. In LmActin, Leu236Asn substitution and the amino acid deletion described above disrupt the hydrophobic contacts and the helical conformation of the loop. Furthermore, the Arg196Thr and Tyr198Thr substitutions in LmActin allow more room for the flexibility of the pointed end tip. c The D-loop of LmActin contains three methionines, from which the Met44 and Met45 insert deep into the adjacent barbed end groove of actin. In contrast, similar extensive hydrophobic insertion is not observed in vertebrate actin.