Fig 1. A working model for fluid-phase and receptor-mediated nutrient uptake through the PVs of G. lamblia.

(A) A TEM image of a PV making contact with a PM-derived invagination (white star). (B) Connections between PVs and ER membranes (white star) are frequently detected in TEM tomograms. (C) The “kiss and flush” working model for fluid-phase and receptor-mediated endocytosis in PVs is based on previously published data [12,13,14,27] and is represented as a continuum of nutrient entry, retention, release, and transfer. (1) Acidifying PVs contain fluid-phase cargo, which is either free or retained by PV-resident receptors (blue ribbons). Released cargo travels further to the lumen of connecting ER tubules. Some cargoes (green pentagon) can be excluded from further passage to the ER. (2) Uptake is mediated by fusion between the VSP-coated PM and the PV membrane through PM-derived invaginations surrounded by clathrin arrays associated to AP2 complexes. This event corresponds to the “kiss” phase in which formation of a channel allows exchange of fluid-phase material between the PV lumen and the extracellular space. Usable nutrients may move freely or be bound by receptors lining the organelle lumen, whereas useless or harmful molecules may be released back in the extracellular space during the “flush” phase. LDL receptor (GILRP) traffics in an AP2-dependent manner from the PM (1) to the PV membrane. (3) PV–PM luminal continuity terminates and a new round of receptor-bound nutrient release mediated by intralumenal acidification ends with further passage to connected ER. (4) The PV is now ready for another round of endocytosis and exchange with the extracellular environment and GILRP is recycled back to the cell’s surface. ER, endoplasmic reticulum; GILRP, giardial putative low-density lipoprotein receptor; LDL, low-density lipoprotein; PM, plasma membrane; PV, peripheral vacuole; TEM, transmission electron microscopy; VSP, variant surface protein.