Biological Activities of Lasso Peptides and Structure–Activity Relationships - Lasso Peptides: Bacterial Strategies to Make and Maintain Bioactive Entangled Scaffolds - page 52

These positions correspond to a nearly continuous surface in the RNAP secondary
channel. In addition, the association between MccJ25 and RNAP was shown by
fluorescence energy transfer (FRET)-binding experiments ( K d 1 µM). The whole
data reported permitted to show that the transcription inhibition by MccJ25 relies
on binding within and obstructing the RNAP secondary channel, generating inter-
ference with NTP uptake and/or binding by RNAP. This mechanism was further
supported by Adelman et al. ( 2004 ) from in vitro studies using biochemical and
single-molecule biophysical approaches. This represents a unique mechanism of
inhibition of RNAP. Kuznedelov et al. (2011) showed that capistruin (Fig. 3.6b ), a
lasso peptide produced by Burkholderia thailandensis E264, also inhibited E. coli
RNAP but not mutant, MccJ25-resistant E. coli RNAP (Kuznedelov et al. 2011 ).
This suggests that RNAP would be a target common to antimicrobial lasso peptides.
The antimicrobial activities of lasso peptides and structure-activity relationships
will be discussed in Sects. 3.1.4 and 3.2, respectively.

3.1.3 HIV Inhibitors

Despite the advances made in the antiretroviral treatment of human immunodefi-
ciency virus (HIV), permitting to halt the replication of HIV and ease AIDS symp-
toms, HIV remains a major public health challenge. HIV replication cycle contains
different key stages that have been targeted by antiretroviral drugs (Richman et al.
2009 ; Moss 2013 ). HIV initiates infection by fusing its envelope membrane with
the host cell membrane (Wilen et al. 2012b , a ; Melikyan 2014 ). The fusion pro-
cess is triggered through sequential interactions between the virus envelope glyco-
protein gp120 with the host cell protein CD4 and the chemokine receptors CCR5
or CXCR4. The formation of the ternary complex gp120-CD4-CCR5 (orCXCR4)
leads to a conformational change in gp120 and to dissociation from the transmem-
brane segment gp41, which inserts into the host cell membrane leading to fusion.
These early steps of the viral replication constitute an attractive target for anti-HIV
therapy (Kazmierski et al. 2006 ; Garg et al. 2011 ).
The 21-residue lasso peptides siamycin I (also named NP-06 or MS-271),
siamycin II and RP 71955 (also named aborycin; see siamycin I primary structure in
Fig. 3.4b ), isolated from Streptomyces sp., inhibit HIV fusion and viral replication
in cell culture. These peptides were discovered in the context of screening microbial
extracts for anti-HIV activities. Helynck et al. (1993) discovered RP 71955 through
a fluorescent assay that aimed at finding inhibitors of the HIV protease (Helynck
et al. 1993 ). In 1995, two independent studies reported the discovery of siamycin I
(or NP-06), one through a tetrazolium-based colorimetric assay (MTT) using MT-4
cells, for the detection of anti-HIV compounds (Chokekijchai et al. 1995 ), and the
other through a syncytia inhibition assay, for the detection of HIV fusion inhibition
(Tsunakawa et al. 1995 ). The latter study also reported the discovery of siamycin
II. The three peptides only differ at position 4 or 17 (Val or Ile residue in each case,
see Chap. 2). Chokekijchai et al. confirmed that siamycin I inhibits the formation of