The primary structure of protein S5 from the small subunit of the Escherichia coli ribosome

Abstract

Protein S5 plays an important role in both assembly and function of the small ribosomal subunit. 30 S subunits reconstituted in the absence of protein SS sediment at 28 S and are only 30-50% active in the in vitro poly (U)-directed polyphenylalanine synthesis system. The ability of these depleted particles to bind fMet-tRNA in the presence of AUG and IF-2 is drastically reduced [ 11. These data are in good agreement with antibody-blocking experiments: antiS5 is a strong inhibitor of the translation of natural and synthetic mRNA and exhibits these effects mainly by inhibition of the initiation step [2]. Furthermore, protein S5 is, together with proteins S2 and S9, implied in the association of ribosomal subunits and stimulates the EF-G-dependent GTPase activity [3]. On the other hand, specific antibodies to protein S5 inhibit neither subunit reassociation [4] nor EF-G-dependent GTP-hydrolysis [ 51. Hydrodynamic studies on isolated protein S5 revealed an elongated shape of this protein [6,7]. A more compact structure for S5 was deduced from neutron scattering experiments [8]. Three antibody binding sites have been detected on the 30 S subunit by immune electron microscopy [9]: one of these sites is located in the lower region of the head, a second in the neck region and a third at the upper part of one of the lobes of the small subunit. Protein S5, besides protein S7, differs among various Eschenkhia coli strains [lo], and altered S5 proteins have been detected in a great variety of mutants: (i) In spectinomycin resistant mutants [ 111;