Biological Activities of Lasso Peptides and Structure–Activity Relationships - Lasso Peptides: Bacterial Strategies to Make and Maintain Bioactive Entangled Scaffolds - page 58

in complex with MccJ25, recently published (Mathavan et al. 2014 ), permitted to
delineate the recognition mechanism. Comparison of the MccJ25- and ferrichrome-
bound FhuA structures revealed that MccJ25 and ferrichrome bind at a very similar
location. MccJ25 completely occupies and occludes the FhuA channel. The loop
region of Mcc25 (residues 9-18) shows significant conformational changes upon
FhuA binding, as compared to the NMR structure of the peptide alone (Bayro et al.
2003 ; Rosengren et al. 2003 ; Wilson et al. 2003 ). This further supports that the
integrity of the loop is essential for binding to FhuA. FhuA/MccJ25 complex is
stabilized by hydrogen bonds involving residues Ala3 and His5 from MccJ25.
SbmA is a homodimeric inner-membrane protein of Gram-negative bacteria,
with seven predicted transmembrane domains (Corbalan et al. 2013 ; Runti et al.
2013 ). It is supposed to be a secondary transporter, although its physiological sub-
strates are not known. SbmA has been involved in the uptake of diverse antibi-
otic agents active on bacteria through an intracellular target: bleomycin (Yorgey
et al. 1994 ) and MccB17 (Lavina et al. 1986 ), both containing thiazole and oxazole
moieties, proline-rich antimicrobial peptides (Mattiuzzo et al. 2007 ) and peptide
nucleic acid-peptide conjugates (Ghosal et al. 2013 ). Mutants of E. coli resistant to
MccJ25 have permitted proposing that SbmA is involved in the uptake of MccJ25
(Salomón and Farías 1995 ), and residue His5 of MccJ25 has revealed important for
SbmA-mediated uptake (de Cristóbal et al. 2006 ).
The knowledge on the function of SbmA has recently been broadened, providing
new leads to understand how MccJ25 crosses the inner membrane of Gram-negative
bacteria. SbmA is homologous and exchangeable with BacA, a bacterial protein
required for bacteria/eukaryotic host chronic relationships. BacA plays an essential
role in Rhizobium spp. symbiosis with leguminous plants (Glazebrook et al. 1993 ;
Ichige and Walker 1997 ) and in Brucella abortus pathogenesis of mammals, which
involves bacteria replication in the host macrophages (LeVier et al. 2000 ). The role
of BacA in the rhizobial association relies on lipopolysaccharide synthesis and pep-
tide transport (Ardissone et al. 2011 ). Furthermore, the gene sbmA is adjacent to a
recently found gene yaiW, and the two genes are co-transcribed in E. coli and Salmo-
nella species (Arnold et al. 2014 ). YaiW is a surface-exposed outer-membrane lipo-
protein, which positively affects the uptake of proline-rich peptides (like SbmA), and
a connection between the cellular functions of SbmA and YaiW has been suggested.
Thus, the role of YaiW in MccJ25 uptake remains to be investigated.
Finally, once in the cytoplasm of target bacteria, MccJ25 inhibits RNAP through
obstructing the RNAP secondary channel, generating interference with NTP uptake
and/or binding by RNAP (Delgado et al. 2001 ; Yuzenkova et al. 2002 ; Adelman
et al. 2004 ; Mukhopadhyay et al. 2004 ; see Sect. 3.1.2).
An alternative target of MccJ25 is the membrane respiratory chain, through the
production of reactive oxygen species (Rintoul et al. 2001 ; Bellomio et al. 2007 ;
Chalon et al. 2011 ; Vincent and Morero 2009 ). The respiratory chain of E. coli
contains different dehydrogenases and terminal reductases (or oxidases), which
are linked by quinones (Unden and Bongaerts 1997 ). These proteins generate the
proton motive force. O 2 is the preferred final electron acceptor and represses the
terminal reductases of anaerobic respiration. The inhibitory effect of MccJ25 on