Active lipoprotein precursors in the Gram-positive eubacterium Lactococcus lactis

Area of Science:

• Microbiology
• Molecular Biology
• Bacterial Physiology

Background:

• Lipid-modified proteins are crucial for bacterial cell-environment interactions.
• Signal peptidase II (SPase II) is essential for Gram-negative bacteria like Escherichia coli but not for Gram-positive Bacillus subtilis.
• The functional significance of lipoprotein processing in Gram-positive bacteria remains unclear.

Purpose of the Study:

• To investigate the role of SPase II in lipoprotein processing in the Gram-positive bacterium Lactococcus lactis.
• To determine if alternative processing or functional precursor lipoproteins exist in L. lactis lacking SPase II.
• To assess the impact of SPase II deficiency on essential lipoprotein function and overall bacterial growth.

Main Methods:

• Construction and characterization of an SPase II mutant strain of Lactococcus lactis.
• Analysis of growth characteristics in milk medium.
• Assessment of extracellular protease activity (PrtP) dependent on lipoprotein PrtM.
• Investigation of alternative N-terminal processing of key lipoproteins (PrtM, OppA).

Main Results:

• Lactococcus lactis SPase II mutant cells exhibited normal growth in milk medium.
• The activity of PrtP, essential for casein degradation, was unaffected in the absence of SPase II.
• No alternative processing of pre-PrtM and pre-OppA was detected in the SPase II mutant.
• Essential lipoproteins PrtM and OppA did not require SPase II processing for function.

Conclusions:

• Lipoprotein precursors in Lactococcus lactis are biologically active without SPase II processing.
• SPase II is not essential for the viability or key functions of L. lactis, unlike in E. coli.
• This study demonstrates the functional significance of unprocessed lipoprotein precursors in Gram-positive bacteria.