Abstract
The extracellular matrix protein Emp of Staphylococcus aureus is a secreted adhesin that mediates interactions between the bacterial surface and extracellular host structures. However, its structure and role in staphylococcal pathogenesis remain unknown. Using multidisciplinary approaches, including circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopy, transmission electron (TEM) and immunogold transmission electron microscopy, functional ELISA assays and in silico techniques, we characterized the Emp protein. We demonstrated that Emp and its truncated forms bind to suprastructures in human skin, cartilage or bone, among which binding activity seems to be higher for skin compounds. The binding domain is located in the C-terminal part of the protein. CD spectroscopy revealed high contents of β-sheets (39.58%) and natively disordered structures (41.2%), and TEM suggested a fibrous structure consisting of Emp polymers. The N-terminus seems to be essential for polymerization. Due to the uncommonly high histidine content, we suggest that Emp represents a novel type of histidine-rich protein sharing structural similarities to leucine-rich repeats proteins as predicted by the I-TASSER algorithm. These new findings suggest a role of Emp in infections of deeper tissue and open new possibilities for the development of novel therapeutic strategies.
Highlights
The extracellular matrix protein extracellular matrix binding protein (Emp) of Staphylococcus aureus is a secreted adhesin that mediates interactions between the bacterial surface and extracellular host structures
PCR analysis revealed that the eap and emp genes are present in almost all S. aureus isolates, whereas they have not been found in any S. epidermidis isolates[10,13]
Hundreds of Emp sequences were found in the protein database within various S. aureus strains, and the sequence identity to our Emp protein was almost higher than 90%
Summary
The extracellular matrix protein Emp of Staphylococcus aureus is a secreted adhesin that mediates interactions between the bacterial surface and extracellular host structures. Adhesion to the extracellular matrix of the host tissue is a critical step in S. aureus pathogenesis This process is essentially mediated by adhesins, which are staphylococcal proteins that undergo strong interactions with the extracellular matrix. The extracellular adhesion protein (Eap) and the extracellular matrix binding protein (Emp) are anchorless proteins and belong to the group of SERAMs9,10 Both proteins are essentially regulated by Sae, but their functions in bacterial adhesion remain unclear[11]. Collagen I, a major component of banded fibrils in skin, is a binding substrate in monomolecular form, but it is almost unrecognized by Eap when incorporated into banded fibrils[16] This phenomenon can be explained by the aggregation of matrix macromolecules in situ into suprastructures such as fibrils, microfibrils, filaments, and networks. These results provide the first structural characterization of Emp, contributing to our understanding of the role of this protein in staphylococcal pathogenesis
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