Temperature-dependent RNP conformational rearrangements: analysis of binary complexes of primary binding proteins with 16 S rRNA.

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.