Characterization of the haem-uptake system of the equine pathogen Streptococcus equi subsp. equi.

This research investigates the haem-uptake system in the equine pathogen Streptococcus equi, a system that greatly resembles those found in Streptococcus pyogenes and Streptococcus zooepidemicus, yet shows considerable differences in one of its proteins. The study analyses the proteins of S. equi’s haem-uptake system, observing how their roles and interactions contribute to the functioning of the bacterium, especially under iron-limited conditions.

About Streptococcus equi’s Haem-Uptake System

• Streptococcus equi, the bacterium that causes strangles in horses, has a haem-uptake system similar to those of Streptococcus pyogenes and Streptococcus zooepidemicus. This system comprises two ligand-binding proteins, Shr and Shp, as well as proteins HtsA-C, which resemble an ABC transporter, a component integral to cellular processes like nutrient uptake.
• Despite its similarities with the haem-uptake systems of other bacteria, S. equi’s system stands out due to its truncated Shr protein – reduced by two-thirds compared to its counterparts. This peculiarity led the researchers to study the different roles of the SeShr, SeShp and SeHtsA proteins.

Mutant Analysis and Protein Characterization

• The researchers designed shr, shp and shphtsA knockout mutants to evaluate whether these proteins were expressed in vitro. The tests showed that all three are expressed and their expression increases under iron-limited conditions.
• Moreover, it was found that SeShr is secreted without any membrane- or cell wall-spanning sequences, while both SeShp and SeHtsA were confirmed to reside within the cellular envelope. It was also discovered that neither SeShp nor SeHtsA could secure haem from haemoglobin-haptoglobin complexes, though they were able to bind with haem.
• The investigators found that SeShr exhibits two notable behaviours. It has both haemoglobin and haemoglobin-haptoglobin binding abilities, like the untruncated Shr. At the same time, it lacks the haem- or fibronectin-binding capabilities, which sets it apart.

Notable Findings

• Researchers also found that some residues within and near the transporter (NEAT) domain are required for ligand binding in SeShr, a binding capability that is apparently compromised due to the truncation of NEAT1 in SeShr.
• Remarkably, when haem and haemoglobin were reintroduced to S. equi under iron-deficient conditions, growth rate nearly returned to normal. However, under in vitro conditions, the wild-type and mutant strains showed similar growth rates.

Overall, this research explores the haem-uptake system of Streptococcus equi, highlighting the unique characteristics of its components and their role in the growth and function of the pathogen. The differences and peculiarities in each protein can serve as potential targets for therapeutic approaches to combat infections by this bacterium.