3) Hfq: a double function protein?
Hfq is an 11 kDa homohexameric protein present in approximately half of
sequenced bacterial genomes and in some archaeal species (Table
1 ) (Sun, 2002; Mura et al. , 2013). It was first described as a
host factor required for the replication of the bacteriophage Qβ RNA
(Franze de Fernandez, Hayward and August, 1972). An E. coli cell
contains approximately 10,000 Hfq hexamers, making it one of the most
abundant proteins. The concentration of Hfq in the stationary phase is
controversial in the literature. Although western blot quantification by
Azam and Ishihama, (1999) showed a decrease in Hfq protein during the
stationary phase of growth, other studies agree on a twofold increase in
the protein level during the stationary phase of growth (Tsui, Feng and
Winkler, 1997; Diestra et al. , 2009). 10%-20% of these hexamers
are found in the nucleoid, 50% are membrane-bound fractions and 30%
are localised in the cytoplasm (Azam, Hiraga and Ishihama, 2000; Diestraet al. , 2009). A Hfq monomer is composed of two domains named the
N-terminal region (NTR) and the C-terminal region (CTR) (Figure
1 ). The NTR is highly conserved across bacteria and in E. coli, it is a 65 amino acid long peptide that is structurally related to the
eukaryotic Sm family of proteins (Mura et al. , 2013). The Sm
proteins have been shown to participate in nucleic acid-related
processes, including splicing, telomere replication, RNA decapping and
decay (Wilusz and Wilusz, 2013). The structure of the NTR reveals that
it folds into five β-sheets and one N-terminal α-helix. Six monomers
assemble into a typical toroidal hexameric ring (Figure 1 )
(Sauter, 2003). This toroid can be divided into three parts: the
proximal face, where the α-helix is located, the distal face where most
of the β-sheets are exposed to the solvent and the lateral rim. The CTR
is located at the periphery of the toroid, is predicted to be
unstructured and has been reported to be present only in a subset of
Gram-negative bacteria (Vogel and Luisi, 2011). Disruption of thehfq gene in most bacterial species, including E. coli ,
results in diverse phenotypes, highlighting the pleiotropic functions of
this protein. These include reduced growth rate, virulence, motility and
biofilm formation, as well as modifications in oxidative and osmotic
sensitivity (Tsui, Leung and Winkler, 1994; Sittka et al. , 2007;
Leonard et al. , 2021). However, the requirement for Hfq appears
to be facultative in a few bacteria, most of which are Gram-positive,
such as Bacillus subtilis or Staphylococcus aureus (Jousselin, Metzinger and Felden, 2009) .