G. Alloing et al., 3 HIGHLY HOMOLOGOUS MEMBRANE-BOUND LIPOPROTEINS PARTICIPATE IN OLIGOPEPTIDE TRANSPORT BY THE AMI SYSTEM OF THE GRAM-POSITIVE STREPTOCOCCUS-PNEUMONIAE, Journal of Molecular Biology, 241(1), 1994, pp. 44-58
Oligopeptides are an important source of nutrients, but can serve also
as signals for intercellular communication. Oligopeptide-binding prot
eins seem likely to play a role both in oligopeptide transport and in
communication processes. One such protein, AmiA, has been identified i
n Streptococcus pneumoniae. amiA is the first gene of an operon, ami,
which encodes a multicomponent oligopeptide transporter belonging to t
he family of ABC transporters (or traffic ATPases). This transporter w
as the first system of this type described in Gram-positive bacteria.
To investigate the role and the subcellular location of the putative o
ligopeptide-binding protein in a bacterium devoid of periplasm, AmiA n
ull mutants were first constructed. None was affected for oligopeptide
uptake by the Ami system. Since this apparent dispensability of AmiA
could result from a functional redundancy, we looked for chromosomal g
enes encoding homologues of AmiA. Two homologous genes were identified
by DNA-DNA hybridization at low stringency with an amiA probe. Both g
enes (aliA and aliB) were cloned and shown to encode putative lipoprot
eins highly homologous to AmiA (close to 60% amino acid identity). Exa
mination of all combinations of amiA, aliA and aliB mutations indicate
d that these proteins have overlapping specificities toward oligopepti
des. The triple mutant is as deficient for oligopeptide transport as m
utants in the amiCBE or F genes, which demonstrates that an oligopepti
de-binding component is absolutely required for transport by the Ami s
ystem. Metabolic labelling with [H-3]palmitic acid and cell fractionat
ion were used to demonstrate that the three proteins are indeed membra
ne-bound lipoproteins in S. pneumoniae. This supports our previous hyp
othesis that substrate-binding lipoproteins are functionally equivalen
t to the periplasmic substrate-binding component of ABC transporters o
f Gram-negative bacteria. Finally, the observation that competence for
genetic transformation was drastically reduced in a particular AliB m
utant suggests that oligopeptide sensing is important for triggering c
ompetence.