A Novel Structural Model of the Creatine Transporter Rationalizes its Structural Determinants of Binding

Abstract

Creatine is a crucial metabolite that plays a fundamental role in ATP homeostasis in tissues with high-energy demands. The creatine transporter (CreaT, SLC6A8) belongs to the solute carrier 6 (SLC6) transporters family, and more particularly to the GABA transporters (GATs) subfamily. Understanding the molecular determinants of specificity within the SLC6 transporters in general, and the GATs in particular is very challenging due to the high similarity of these proteins. In the study presented here, our efforts focused on finding key structural features involved in binding selectivity for CreaT using structure-based computational methods. Due to the lack of tridimensional structures of SLC6A8, our approach was based on the realization of a homology model combining the structures of two templates, i.e. the human serotonine transporter and the prokaryotic leucine transporter. During this meticulous process, particular attention was given to the transmembrane helix 10, in which previous studies identify a π-helix among the GATs subfamily. Understanding the role of the π-helix and its function in the binding site of CreaT is of outmost importance for the realization of a reliable homology model and thus our primary concern. Our model reveals that an optimal complementarity between the shape of the binding site and the size of the ligands is necessary for transport. These findings provide a framework for a deeper understanding of substrate selectivity of the SLC6 family and other LeuT fold transporters.