Breaking barriers: pCF10 type 4 secretion system relies on a self-regulating muramidase to modulate the cell wall

Area of Science:

• Microbiology
• Molecular Biology
• Biochemistry

Background:

• Conjugative type 4 secretion systems (T4SSs) are crucial for the spread of antibiotic resistance and virulence factors in bacteria.
• Understanding Gram-positive T4SSs is vital, as they are implicated in numerous hospital-acquired infections.

Purpose of the Study:

• To elucidate the function and regulation of PrgK, a key cell wall hydrolase in the Gram-positive enterococcal conjugative plasmid pCF10 T4SS.
• To characterize the enzymatic activities of PrgK's three predicted extracellular hydrolase domains: LytM, soluble lytic transglycosylase (SLT), and CHAP.

Main Methods:

• Structural analysis of the LytM domain.
• In vitro enzymatic assays to determine the activity of the SLT domain.
• Investigating the regulatory roles of LytM and CHAP domains on SLT activity.
• Assessing the role of the CHAP domain in PrgK dimerization.
• Identifying interactions between PrgK and other T4SS components, such as PrgL.

Main Results:

• The LytM domain's active site is degenerate and lacks the necessary metal cofactor.
• The SLT domain exhibits unexpected muramidase activity, not lytic transglycosylase activity.
• LytM and CHAP domains downregulate the SLT muramidase activity.
• The CHAP domain is essential for PrgK dimer formation.
• PrgK interacts with PrgL, likely facilitating its targeting within the T4SS.

Conclusions:

• PrgK's functional domains and their regulatory interactions are critical for Gram-positive T4SS function.
• The unexpected muramidase activity of the SLT domain and the regulatory roles of LytM and CHAP provide new insights into cell envelope hydrolysis during conjugation.
• This study enhances the understanding of horizontal gene transfer mechanisms in *Enterococcus faecalis* and other medically relevant Gram-positive bacteria.