The two tryptophans of β2-microglobulin have distinct roles in function and folding and might represent two independent responses to evolutionary pressure
- Equal contributors
1 Department of Biochemistry, University of Pavia, via Taramelli 3b, 27100 Pavia, Italy
2 Department of Computer Engineering and Systems Science, University of Pavia, via Ferrata 1, 27100 Pavia, Italy
3 Department of Biomedical Sciences and Technologies, University of Udine, P.le Kolbe 4, 33100 Udine, Italy
4 Department of Biomolecular Sciences and Biotechnology, CNR-INFM and CIMAINA, University of Milano, Via Celoria 26, 20133-Milan, Italy
5 Department of Internal Medicine and Medical Therapy, University of Pavia, viale Golgi 19, 27100 Pavia, Italy
6 Department of Organic Chemistry and Biochemistry, University of Naples "Federico II" and CEINGE-Biotecnologie Avanzate, via Cinthia 6, 80126 Naples, Italy
BMC Evolutionary Biology 2011, 11:159 doi:10.1186/1471-2148-11-159Published: 10 June 2011
We have recently discovered that the two tryptophans of human β2-microglobulin have distinctive roles within the structure and function of the protein. Deeply buried in the core, Trp95 is essential for folding stability, whereas Trp60, which is solvent-exposed, plays a crucial role in promoting the binding of β2-microglobulin to the heavy chain of the class I major histocompatibility complex (MHCI). We have previously shown that the thermodynamic disadvantage of having Trp60 exposed on the surface is counter-balanced by the perfect fit between it and a cavity within the MHCI heavy chain that contributes significantly to the functional stabilization of the MHCI. Therefore, based on the peculiar differences of the two tryptophans, we have analysed the evolution of β2-microglobulin with respect to these residues.
Having defined the β2-microglobulin protein family, we performed multiple sequence alignments and analysed the residue conservation in homologous proteins to generate a phylogenetic tree. Our results indicate that Trp60 is highly conserved, whereas some species have a Leu in position 95; the replacement of Trp95 with Leu destabilizes β2-microglobulin by 1 kcal/mol and accelerates the kinetics of unfolding. Both thermodynamic and kinetic data fit with the crystallographic structure of the Trp95Leu variant, which shows how the hydrophobic cavity of the wild-type protein is completely occupied by Trp95, but is only half filled by Leu95.
We have established that the functional Trp60 has been present within the sequence of β2-microglobulin since the evolutionary appearance of proteins responsible for acquired immunity, whereas the structural Trp95 was selected and stabilized, most likely, for its capacity to fully occupy an internal cavity of the protein thereby creating a better stabilization of its folded state.