Fig. 4. Structure of the RcGTA tail.

a The tube of the RcGTA tail is formed by the stopper, tail terminator, tail tube, and distal tail proteins. On the right side, the proteins are shown in cartoon representations with β-strands forming the core of the proteins in yellow, N-termini in green, short loops in orange, long loops in magenta, insertion loops in cyan, and central helices in blue. b Structural similarity (upper left) and sequence identity (bottom right) of RcGTA tail proteins. Z-scores were calculated using the DALI server⁷⁵. Values higher than two indicate that the compared proteins are similar. c Superposition of hexamers of stopper proteins of RcGTA, colored as in a, and phage SPP1 (PDB 5A20_EF), in gray. The sidechains of residues that form the bottlenecks in tails of RcGTA and SPP1 are shown in stick representation. The long loop of the stopper protein of RcGTA (magenta) does not reach as close to the center of the channel as that of SPP1. d Central slice through cryo-EM map of RcGTA tail. The parts of the density belonging to RcGTA proteins are color-coded as in panel a. The density in the central channel is color-coded according to the domains of tape measure protein shown in panel f. e Cryo-EM map of RcGTA tail with fitted tail proteins and tail-needle protein of P22 in cartoon representation. The P22 tail-needle model is color-coded according domains shown in panel f. f Tape measure protein of RcGTA is structurally similar to tail-needle protein (PDB 2POH) of phage P22 from the family Podoviridae. Diagrams of secondary structure elements of the two proteins are shown. α-helices are indicated by wiggly lines and β-strands by broad colored lines. The N-terminal region (grey), responsible for the attachment of the needle protein to the tip of P22 tail, is missing from the RcGTA protein. The gray rectangle indicates the position of the sequence displayed in panel g. g Sequence and secondary structure alignment of 41 residues from coiled-coil regions of the RcGTA tape measure protein and phage P22 tail-needle protein computed using HHpred⁴⁴.