Compositional and structural features related to thermal stability in the archaea SRP19 and SRP54 signal recognition particle proteins.

To survive at high temperature, thermophile organisms must adapt their biomolecules. In both nucleic acids and proteins, this adaptation involves a vast array of compositional and structural modifications. The archaea stand out as the only group of organisms that have species capable of growing at temperatures ranging from 0 to 110°C. In this study, we have used the archaea genome datasets to identify molecular trends related to thermal adaptation in the protein components (SRP19 and SRP54) of the signal recognition particle (SRP). Using comparative genomics and secondary structure homology modeling we have detected significant differences in the amino acids composition and distribution between the SRP proteins of thermophile and mesophile archaea. These include: a significant increase in the thermophile SRP proteins of the frequency of charged amino acids able to participate in electrostatic interactions which contribute to stabilize proteins; decreased content of both thermolabile and small/tiny amino acids which usually contribute to protein flexibility; and a significant increase in aliphatic and aromatic amino acids providing good covering and masking to produce hydrophobic pockets involved in stabilizing protein structure. Moreover, a detailed analysis of the four structural and functional domains of the SRP54 indicates a particularly robust correlation between the compositional properties of the M domain and the optimal growth temperature (OGT) of the archaea. The analysis of the bacterial SRP54(Ffh) shows similar adaptations to the OGT. Thus, natural selection has adapted the SRP proteins to the OGT of the archaea and bacteria species by modifying both, their amino acids composition and distribution.