Biological and structural characterization of the Type 3 fimbrial subunit <scp>MrkA</scp> from <scp> <i>Klebsiella pneumoniae</i> </scp>

Klebsiella pneumoniae is a Gram-negative opportunistic pathogen responsible for a wide range of community-associated and hospital-acquired infections and a major cause of neonatal sepsis in low- and middle-income countries. The pathogen's surface fimbriae, particularly the Type 3 fimbriae, are critical for bacterial adhesion, biofilm formation, and host defense evasion. MrkA, the pathogen's major Type 3 fimbrial subunit, has a structural function in fimbrial assembly, but its three-dimensional structure remains to be fully elucidated. In this study, we utilized solution-state nuclear magnetic resonance spectroscopy to elucidate the structure of MrkA, leveraging previously reported chemical shift assignments of a designed self-complementing monomeric protein. Additionally, we confirmed the ability of monoclonal antibodies, capable of recognizing MrkA oligomers on wild-type Klebsiella bacteria, to bind the recombinant MrkA protein. Our findings contribute to the evaluation of MrkA as a potential target in vaccine development against Klebsiella pneumoniae infections.