Science 6.03.2020

a lack of transport of PE19. The phylogeny and
distribution of the PE/PPE proteins is such
that all slow-growing mycobacteria seem to
have an expanded repertoire of heterodimer
combinations, and these species lack any de-
tectable homolog of the porin seen in the fast-
growing mycobacteria that have few PPEs
(table S4). Our data, coupled with the dis-
tribution of PE/PPE among the mycobacteria,
suggest that slow-growing mycobacteria have
co-opted at least some of these proteins as
small molecule–selective channels in place of
porins. The need for these porins is intimately
associated with the production of PDIM. De-
spite the fact that PDIM accounts for as
much as 46% of the total lipids present in
M. tuberculosisand for 11% of the total bac-
terial mass in culture ( 21 ), its molecular role in
theenvelopehasbeenunclear.PDIM,likethe
PE/PPE proteins, is also restricted in distribu-
tion to the slow-growing mycobacteria, and
our results indicate that its role is to enhance
the impermeability of the mycobacterial cell
wall. Fully 1% of the coding capacity of the
organism is devoted to the biosynthesis of
PDIM, and PDIM-deficient mutants are highly
attenuated in virulence ( 30 , 31 ). PDIM is easily
lost during in vitro culture yet never absent
from freshly isolated clinical strains ( 20 ). Our
results suggest that PDIM acts within the
outer leaflet of the mycobacterial cell envelope
to enhance its impermeability, presumably
thereby directly allowing growth in the hostile
environment of the host. At least some of the

PE/PPE proteins appear to act as solute-

selective pores, allowingM. tuberculosisaccess

to nutrients required to proliferate in this

environment. Understanding the transport

properties and selectivity of these particular

channels would enable rational approaches to

optimize the uptake properties of newly de-

signed drugs.

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ACKNOWLEDGMENTS

We thank DuPont Agriculture for providing the library of

compounds that included 3bMP1. We also thank G. Prosser

and K. Kolbe for the modification of plasmids used in this study,

and A. Bohrer for help with flow-cytometry assays.Funding:

This work was funded, in part, by the Intramural Research Program

of the NIAID, NIH, and by the Bill and Melinda Gates Foundation

through the TB Drug Accelerator program.Author contributions:

Q.W., J.R.H., P.C.R., and S.R.G. performed the experiments.

P.G.W., H.I.M.B., and C.E.B. designed the experiments. Q.W.,

H.I.M.B., and C.E.B. wrote the manuscript.Competing interests:

The authors declare no competing interests.Data and materials

availability:All data are available in the main text or the

supplementary materials.

SUPPLEMENTARY MATERIALS

science.sciencemag.org/content/367/6482/1147/suppl/DC1

Materials and Methods

Figs. S1 to S10

Tables S1 to S6

References ( 32 – 42 )

View/request a protocol for this paper fromBio-protocol.

9 October 2018; resubmitted 31 October 2019

Accepted 5 February 2020

10.1126/science.aav5912

Wanget al.,Science 367 , 1147–1151 (2020) 6 March 2020 5of5

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