Temperature-dependent RNP conformational rearrangements: analysis of binary complexes of primary binding proteins...

Ribonucleoprotein particles (RNPs) are important components of all living systems, and the assembly of these particles is an intricate, often multistep, process. The 30 S ribosomal subunit is composed of one large RNA (16 S rRNA) and 21 ribosomal proteins (r-proteins). In vitro studies have revealed that assembly of the 30 S subunit is a temperature-dependent process involving sequential binding of r-proteins and conformational changes of 16 S rRNA. Additionally, a temperature-dependent conformational rearrangement was reported for a complex of primary r-protein S4 and 16 S rRNA. Given these observations, a systematic study of the temperature-dependence of 16 S rRNA architecture in individual complexes with the other five primary binding proteins (S7, S8, S15, S17, and S20) was performed. While all primary binding r-proteins bind 16 S rRNA at low temperature, not all r-proteins/16 S rRNA complexes undergo temperature-dependent conformational rearrangements. Some RNPs achieve the same conformation regardless of temperature, others show minor adjustments in 16 S rRNA conformation upon heating and, finally, others undergo significant temperature-dependent changes. Some of the architectures achieved in these rearrangements are consistent with subsequent downstream assembly events such as assembly of the secondary and tertiary binding r-proteins. The differential interaction of 16 S rRNA with r-proteins illustrates a means for controlling the sequential assembly pathway for complex RNPs and may offer insights into aspects of RNP assembly in general.