STOICHIOMETRY AND DOMAINAL ORGANIZATION OF THE LONG TAIL-FIBER OF BACTERIOPHAGE-T4 - A HINGED VIRAL ADHESIN

The long-tail fibers (LTFs) form part of bacteriophage T4's apparatus for host cell recognition and infection, being responsible for its ini tial attachment to susceptible bacteria. The LTF has two parts, each s imilar to 70 to 75 nm long; gp34 (140 kDa) forms the proximal half-fib er, while the distal half-fiber is composed of gp37 (109 kDa), gp36 (2 3 kDa) and gp35 (30 kDa). LTFs have long been thought to be dimers of gp34,gp37 and gp36, with one copy of gp,35. We have used mass mapping by scanning transmission electron microscopy (STEM), quantitative SDS- PAGE, and computational sequence analysis to study the structures of p urified LTFs and half-fibers of both kinds. These data establish that the LTF is, in fact, trimeric, with a stoichiometry of gp34: gp37: gp3 6: gp35 = 3:3:3:1. Averaged images of stained and unstained molecules resolve the LTF into a linear stack of 17 domains. At the proximal end is a globular domain of similar to 145 kDa that becomes incorporated into the baseplate. It is followed by a rod-like shaft (33 x 4 nm; 151 kDa) which correlates with a cluster of seven quasi repeats, each 34 to 39 residues long. The proximal half-fiber terminates in three globu lar domains. The distal half-fiber consists of ten globular domains of variable size and spacing, preceding a needle-like end domain (15 x 2 .5 nm; 31 kDa). The LTF is rigid apart from hinges between the two mos t proximal domains, and between the proximal and distal half-fibers. T he latter hinge occurs at a site of local non-equivalence (the ''kneec ap'') at which density, correlated with the presence of gp35, bulges a symmetrically out on one side. Several observations indicate that gp34 participates in the sharing of conserved structural modules among col iphage tail-fiber genes to which gp37 was previously noted to subscrib e. Two adjacent globular domains in the proximal half-fiber match a pa ir of domains in the distal half-fiber, and the rod domain in the prox imal half-fiber resembles a similar domain in the T4 short tail-fiber (gp12). Finally, possible structures are considered; combining our dat a with earlier observations, the most likely conformation for most of the LTF is a three-stranded beta-helix. (C) 1996 Academic Press Limite d