The evolutionary history of topological variations in the CPA/AT transporters

Functional insight into Cordyceps militaris sugar transporters by structure modeling, network analysis and allosteric regulation

Insights into the structures, functions and dynamics of Cordyceps militaris (C. militaris) sugar transporters are necessary for understanding their versatile metabolic capability for fungal growth. The sequence-function relationship study of 85 C. militaris sugar transporters showed that there is a gap between phylogenetic-based subfamily classification and their functions. Beyond protein sequences, structural modeling and principal component analysis of the structural ensemble revealed the different folds of the Car and Org subfamilies. Performing channel detection and network analysis found that the Alp and Hex subfamilies can be specifically distinguished from others by the betweenness of channel residues. Signature dynamics analysis further suggested that the Hex subfamily demonstrates different dynamics, with high flexibility at the H1 region in TM11. Furthermore, the H1 region as an allosteric site was examined by network parameter calculations that guided allosteric pathways between this region and the channel cavity. Together with gene expression data of C. militaris, e.g., Hex06741 in the Hex subfamily, it was promisingly expressed when sugar utilization was altered. This work demonstrates an in silico framework for investigating C. militaris sugar transporters as an example case study of the allosteric activity of the Hex subfamily and can facilitate sugar transporter engineering design that can further optimize the preferable sugar utilization and fermentation process of C. militaris.

High-speed atomic force microscopy reveals a three-state elevator mechanism in the citrate transporter CitS

As cellular membranes are impermeable to most molecules, transporter proteins are typically present in the membrane to transport small molecules in or out of the cell. Due to the small, nanometer size of these transporters, it is challenging to study their transport mechanism. Here, we use advanced microscopy approaches to study in real time and at the single-molecule level the mode of action of the dimeric CitS tranpsorter. Using high-speed atomic force microscopy, we visualize the dynamic, elevator-like movement of the transporter, and we reveal that the two protomers move independently. We also discovered an intermediate state, reminiscent of another, unrelated transporter, indicating that independent evolutionary pathways have led to similar three-state elevator mechanisms.

The secondary active transporter CitS shuttles citrate across the cytoplasmic membrane of gram-negative bacteria by coupling substrate translocation to the transport of two Na⁺ ions. Static crystal structures suggest an elevator type of transport mechanism with two states: up and down. However, no dynamic measurements have been performed to substantiate this assumption. Here, we use high-speed atomic force microscopy for real-time visualization of the transport cycle at the level of single transporters. Unexpectedly, instead of a bimodal height distribution for the up and down states, the experiments reveal movements between three distinguishable states, with protrusions of ∼0.5 nm, ∼1.0 nm, and ∼1.6 nm above the membrane, respectively. Furthermore, the real-time measurements show that the individual protomers of the CitS dimer move up and down independently. A three-state elevator model of independently operating protomers resembles the mechanism proposed for the aspartate transporter Glt_Ph. Since CitS and Glt_Ph are structurally unrelated, we conclude that the three-state elevators have evolved independently.

The evolutionary history of topological variations in the CPA/AT transporters

CPA/AT transporters are made up of scaffold and a core domain. The core domain contains two non-canonical helices (broken or reentrant) that mediate the transport of ions, amino acids or other charged compounds. During evolution, these transporters have undergone substantial changes in structure, topology and function. To shed light on these structural transitions, we create models for all families using an integrated topology annotation method. We find that the CPA/AT transporters can be classified into four fold-types based on their structure; (1) the CPA-broken fold-type, (2) the CPA-reentrant fold-type, (3) the BART fold-type, and (4) a previously not described fold-type, the Reentrant-Helix-Reentrant fold-type. Several topological transitions are identified, including the transition between a broken and reentrant helix, one transition between a loop and a reentrant helix, complete changes of orientation, and changes in the number of scaffold helices. These transitions are mainly caused by gene duplication and shuffling events. Structural models, topology information and other details are presented in a searchable database, CPAfold (cpafold.bioinfo.se).