By Diana Silva Brenes
Relating molecular structure to function is the first step and one of the greatest challenge to understand nature’s designs or to make novel “functional designs” of our own. This paper by the Chilkoti group begins with statistical analysis of some of the most relevant proteins displaying LCST and UCST behavior. By analyzing the peptide sequences, the authors identify as common motif for both behaviors a high glycine & proline content. Furthermore, for LCST abundance of aromatic residues seems to be a requirement whereas UCST peptides seem to be encoded by a pair of zwitterionic residues.
To test if these observations lead to LCST/UCST phenomena, over 80 model peptides were recombinantly synthesized and their thermoresponsive behavior was measured by UV absorbance while changing the temperature. Each peptide presented the predicted behavior, giving support to their observations. Furthermore, by comparing a few selected examples, they show how an increase in hydrophobicity leads to an increased UCST cloud point and how eliminating one of the residues from azwitterionic pair turns a UCST peptide to an LCST peptide.
The LCST and UCST behavior is, however, a complex phenomenon dependent on protein-protein versus protein-water interactions, which in turn are modulated by more factors aside from the sequence of the protein. The possible scenarios are limitless, and the authors give insight on the most significant: peptide length, concentration, and pH (charge state of protonable atoms).
The robustness of the behavior encoded in the rules they found can be seen by a hybrid peptide containing both an LCST portion and a UCST one. The resulting peptide displays both behaviors, albeit at different temperatures from the “parent” sequences.
Finally, the authors show that searching for the characteristics they determined as important for LCST/UCST behavior throughout the human proteome produces examples of proteins whose function could very well be related to a thermoresponsive behavior, highlighting the applicability of their observations to understand the phenomena that make life as we know it possible.
Quiroz, 2015. Sequence heuristics to encode phase behaviour in intrinsically disordered protein polymers