Nature | Vol 578 | 13 February 2020 | 321Article
The molecular basis for sugar import in
malaria parasites
Abdul Aziz Qureshi1,3, Albert Suades1,3, Rei Matsuoka^1 , Joseph Brock^1 , Sarah E. McComas1,2,
Emmanuel Nji^1 , Laura Orellana^1 , Magnus Claesson^1 , Lucie Delemotte^2 & David Drew^1 *Elucidating the mechanism of sugar import requires a molecular understanding of
how transporters couple sugar binding and gating events. Whereas mammalian
glucose transporters (GLUTs) are specialists^1 , the hexose transporter from the malaria
parasite Plasmodium falciparum PfHT1^2 ,^3 has acquired the ability to transport both
glucose and fructose sugars as efficiently as the dedicated glucose (GLUT3) and
fructose (GLUT5) transporters. Here, to establish the molecular basis of sugar
promiscuity in malaria parasites, we determined the crystal structure of PfHT1 in
complex with d-glucose at a resolution of 3.6 Å. We found that the sugar-binding site
in PfHT1 is very similar to those of the distantly related GLUT3 and GLUT5 structures^4 ,^5.
Nevertheless, engineered PfHT1 mutations made to match GLUT sugar-binding sites
did not shift sugar preferences. The extracellular substrate-gating helix TM7b in
PfHT1 was positioned in a fully occluded conformation, providing a unique glimpse
into how sugar binding and gating are coupled. We determined that polar contacts
between TM7b and TM1 (located about 15 Å from d-glucose) are just as critical for
transport as the residues that directly coordinate d-glucose, which demonstrates a
strong allosteric coupling between sugar binding and gating. We conclude that PfHT1
has achieved substrate promiscuity not by modifying its sugar-binding site, but
instead by evolving substrate-gating dynamics.P. falciparum relies on a continuous supply of host-derived glucose
during the clinically important asexual stages of growth and replica-
tion within erythrocytes^6. As a consequence, glucose consumption is
increased about 100-fold in erythrocytes that contain the parasite^7.
This metabolism is further dependent upon the import of glucose
across the cell membrane of the parasite by the hexose transporter
PfHT1^2 ,^3 (Fig. 1a). Owing to its essential role in glucose metabolism,
PfHT1 is a well-recognized target for antimalarial drugs^8 –^11. PfHT1
belongs to the major facilitator superfamily (MFS), the members of
which share a fold that consists of two symmetrical six transmem-
brane (TM) bundles of helices^12 ,^13 —as was first clearly observed in
the structure of lactose permease (LacY)^14. However, PfHT1 clusters
with a separate MFS class than that of LacY; it belongs to the subfam-
ily of sugar porters^15 ,^16 , which includes the medically relevant GLUT
transporters^1. In contrast to the GLUT transporters (which show poor
catalytic activity for diverse sugars^1 ), PfHT1 shows a broader substrate
specificity^2 ,^17. In particular, PfHT1 has acquired the ability to transport
both d-glucose and d-fructose with kinetics (KM) similar to those of
the dedicated high-affinity d-glucose (GLUT3) and d-fructose (GLUT5)
transporters, respectively^1 –^3. Structures of the related sugar porters
GLUT1^18 , GLUT3^4 and GLUT5^5 , as well as the Escherichia coli xylose
transporter XylE^19 –^21 , have previously been determined^22 ,^23. Because
PfHT1 shares only low sequence identity with these transporters
(Extended Data Fig. 1a, b), it has been unclear whether sugar recog-
nition would be similar. Here we aimed to determine the structure
of PfHT1 to establish the molecular ‘rules’ that govern its substrate
specificity and inhibition.
Purified PfHT1 was reconstituted into liposomes and showed robust
uptake of radiolabelled d-glucose, d-mannose, d-galactose, d-fructose
and d-xylose, consistent with in vivo analysis^3 (Fig. 1b, Extended Data
Fig. 2a–e). PfHT1 can also transport d-glucosamine (Extended Data
Fig. 2f ), as has previously been observed for GLUT2 and the galactose
transporter GalP^24 ,^25. PfHT1 kinetics for d-glucose (KM of 0.80 mM) and
d-fructose (KM of 9.6 mM) in proteoliposomes was comparable to in vivo
estimates for PfHT1, GLUT3 and GLUT5^1 ,^3 ,^26 ,^27 (Extended Data Fig. 3a, d).
The turnover rates (kcat) for d-glucose (19 s−1) and d-fructose (30 s−1) were
further comparable to in vitro estimates for those of GLUT3 (13 s−1)^28
and GLUT5 (43 s−1), respectively (Extended Data Table 1). We co-crys-
tallized PfHT1 with d-glucose using the vapour-diffusion method, and
determined the structure by molecular replacement at a resolution
of around 3.6 Å (Fig. 1c, Extended Data Table 2). PfHT1 crystallized as
a dimer, with four molecules in the asymmetric unit (Extended Data
Fig. 4a). The PfHT1 structure is highly similar to the outward-occluded
structure of human GLUT3^4 (Extended Data Fig. 4b). The extracellu-
lar half-helix TM7b in PfHT1 was found to be more occluded than in
human GLUT3, and matched the position of TM7b in the inward-open
conformation of GLUT1 and GLUT5 (Fig. 1d, Extended Data Fig. 4d).
Nonetheless, PfHT1 was not in an inward-facing state as access to the
inside was closed. We conclude PfHT1 has been captured in a previously
unobserved, fully occluded conformation (Fig. 1c, e).https://doi.org/10.1038/s41586-020-1963-z
Received: 17 June 2019
Accepted: 3 January 2020
Published online: 29 January 2020
(^1) Department of Biochemistry and Biophysics, Stockholm University, Stockholm, Sweden. (^2) Department of Applied Physics, Science for Life Laboratory, KTH Royal Institute of Technology,
Stockholm, Sweden.^3 These authors contributed equally: Abdul Aziz Qureshi, Albert Suades. *e-mail: [email protected]