Effect of ribonuclease T1 on ribosomal subunits of rat liver

Structural differences between active and inactive mammalian 60S ribosomal subunits. Circular dichroism and electric birefringence studies

The structure and conformation of different active and inactive forms of the 60S rat liver ribosomal subunits have been analyzed by electric birefringence and circular dichroism. These studies show the following: 1) When a phosphate buffer is used instead of a triethanolamine buffer, there are major changes in RNA stacking, RNA-protein interactions, and particle orientation and conformation with no concomitant loss in ribosome activity. 2) The inactivated subunits by K(+)-depletion exhibit the same electro-optical and near-UV CD behaviour than the active subunits in phosphate buffer. 3) Inactivation by EDTA-treatment leads to drastic changes in RNA structure, RNA-protein interactions and subunit conformation; the 60S particles behave like free RNA, indicating the absence of any stabilization of rRNA by ribosomal proteins. 4) The inactivation of subunits by depletion of either monovalent or divalent cations is accompanied by a net decrease of the alpha-helicity of the ribosomal proteins. 5) The transition from active to inactive form of 60S subunits may involve protein modifications, likely dependent on a specific array of cations. 6) RNA has a certain degree of liberty within the subunits and one can suppose that this property is responsible for the flexible structure of ribosome.

Electric birefringence of eukaryotic ribosomes

Eukaryotic 60S ribosomal subunits were studied by transient electric birefringence. Conformations of subunits in active and inactive states upon changes in magnesium concentration were compared by electric birefringence and orientational relaxation time measurements. Active subunits exhibit a positive birefringence and a relaxation time of the order of 8 microseconds. In the presence of EDTA, inactive subunits show no birefringence. When Mg2+ is reverted in the cold to its initial level, the electro-optical properties of the subunits are partially restored although the particles remain biologically inactive.

Two specific ribonucleoprotein fragments from rat liver 60S ribosomal subunits

K+-depleted 60S ribosomal subunits from rat liver were submitted to a mild treatment with ribonuclease T1. Ribonucleoprotein fragments could be separated on sucrose gradients only when the digested subunits were partially deproteinized with a high KCl concentration (0.6 M) which removed seven proteins more or less completely and 5S RNA. The RNA and protein content of each fragment has been characterized. The largest ribonucleoprotein enclosed two RNA fragments of about 950,000 and 750,000 daltons and all the salt-resistant proteins except L5. The smallest one enclosed protein L5 (with L11, L17 and L26 in small amounts) and a 67,000 RNA piece. The subsequent hydrolysis of the large ribonucleoprotein produced several other ribonucleoproteins. One of them has been fully characterized: it enclosed a 250,000 RNA fragment and protein L12 (with L11, L25 and L30 in smaller amounts).

An argument for the existence of a natural complex between protein L5 and 5 S RNA in rat liver 60-S ribosomal subunits

Electrophoresis of the 60-S ribosomal subunits from rat liver in the presence of citrate ions removes the 7 S ribonucleoprotein complex between protein L5 and 5 S RNA though this complex is not released by dialysis or by centrifugation through a sucrose cushion in the same buffer. Using acetate instead of citrate, the subunits remain intact in all cases. On the other hand, in the presence of EDTA, the complex is always released. The poly(U) directed polyphenylalanine synthesis is correlated in each case with the presence of this complex within the subunits. The melting curves of subunits which have been treated with citrate, acetate or EDTA and then taken back in the buffer in which they were stored suggest that the specific RNA-protein interactions are preserved in the presence of acetate and of citrate but not of EDTA. As a whole, the results support the interpretation that the association of protein L5 and 5 S RNA exists within the active subunits.

Comparison of the protein content of free and membrane-bound rat liver polysomes and of the derived subunits

Ribosomal proteins from pure free and membrane-bound rat liver polysomes were analyzed with a highly resolutive two-dimensional gel electrophoresis technique, using sodium dodecyl sulfate in the second dimension. Three acidic proteins found in free polysomes were always absent from the membrane-bound polysomes. Their molecular weights were estimated to be 20 000, 19 500 and 18 500. When free ribosomes were dissociated into subunits, the three protein spots were still found in the 60S subunit pattern, but they were weaker than in polysomes. A possible involvement of these three proteins in the attachment of ribosomal structures to the membranes is proposed.