De. Nelson et Kd. Young, Contributions of PBP 5 and DD-carboxypeptidase penicillin binding proteinsto maintenance of cell shape in Escherichia coli, J BACT, 183(10), 2001, pp. 3055-3064
Escherichia coli has 12 recognized penicillin binding proteins (PBPs), four
of which (PBPs 4, 5, and 6 and DacD) have DD-carboxypepidase activity. Alt
hough the enzymology of the DD-carboxypeptidases has been studied extensive
ly, the in vivo functions of these proteins are poorly understood. To expla
in why E. coli maintains four independent loci encoding enzymes of consider
able sequence identity and comparable in vitro activity, it has been propos
ed that the DD-carboxypeptidases may substitute for one another in vivo. We
tested the validity of this equivalent substitution hypothesis by investig
ating the effects of these proteins on the aberrant morphology of Delta dac
4 mutants, which produce no PBP 5, Although cloned PBP 5 complemented the m
orphological phenotype of a Delta dacA mutant lacking a total of seven PBPs
, controlled expression of PBP 4, PBP 6, or DacD did not. Also, a truncated
PBP 5 protein lacking its amphipathic C-terminal membrane binding sequence
did not reverse the morphological defects and was lethal at low levels of
expression, implying that membrane anchoring is essential for the proper fu
nctioning of PBP 5. By examining a set of mutants from which multiple PBP g
enes were deleted, we found that significant morphological aberrations requ
ired the absence of at least three different PBPs. The greatest defects wer
e observed in cells lacking, at minimum, PBPs 5 and 6 and one of the endope
ptidases (either PBP 4 or PBP 7). The results further differentiate the rol
es of the low-molecular-weight PBPs, suggest a functional significance for
the amphipathic membrane anchor of PBP 5 and, when combined with the recent
ly determined crystal structure of PBP 5, suggest possible mech anisms by w
hich these PBPs may contribute to maintenance of a uniform cell shape in E.
coli.