FC0104
Gelatinase-associated lipocalin receptor (LCN2-R, SLC22A17)  -  Lipocalin (NGAL)

Biological function
LCN2-R is the cellular receptor of the mammalian lipocalin (NGAL) and promotes endocytosis of both its iron-bound and iron- free forms. As part of the antibacterial innate response, NGAL binds the bacterial siderophore enterobactin with high affinity and sequesters enterobactin during a bacterial infection. This withdraws the siderophore from the bacterium thereby limiting its access to environmental iron.

Domain organization/sequence features
LCN2-R belongs to the SLC22 family of organic ion transporters that consist of 12 transmembrane (TM) helical segments organized in two bundles of six TMs each connected by a large intracellular loop. LCN2-R has an unusual topology, as first 100 residues (hLCN2-R-NTD) does not form a TM, instead they are part of an extracellular soluble domain containing two N- glycosylation sites followed by a first bundle of five TMs, a large intracellular loop, and final bundle of six TMs, and an intracellular C-terminal domain.

Structural evidence
¹H,¹⁵N HSQC spectrum of hLCN2-R-NTD devoid of any stable secondary or tertiary structures and indicates significant conformational exchange on the micro-to-millisecond timescale. Upon interactions with NGAL only very limited chemical shift changes can be observed, which is strongest for the C-terminal part of hLCN2-R-NTD where most of the resonances completely vanished upon the addition of only 0.3 eq of NGAL. Most resonances remain visible even in presence of 2 eq. of unlabeled NGAL, although the N-terminal part seems more affected than the middle of the protein. The reciprocal experiment with unlabeled hLCN2-R-NTD and labeled NGAL provided the same result. Paramagnetic relaxation enhancement of the NGAL-hLCN2-R-NTD complex shows that the N-terminal part of hLCN2-R-NTD is significantly affected, owing to its spatial proximity to the C-terminal part, and also with NGAL in the complex. An enhanced intensity decrease was observed for the middle part of the protein, which indicates transient interactions with the paramagnetic NGAL. Exchange rate obtained from relaxation dispersion experiments (k_ex 1200-1300 s^-1) are consistent with complex series of binding events.

Biochemical evidence
hLCN2-R-NTD binds to NGAL with an affinity (K_D) of 10 μM in 1:1 stoichiometry. The thermodynamic parameters of binding indicate that the binding is enthalpically driven but includes a non-negligible entropic contribution (ΔH = -4.5 kcal/mol ^-1 ; ΔTΔS = -2.3 kcalmol^-1 at 25 °C). hLCN2-R-NTD does not, or poorly bind to holo-NGAL.

Structure/Mechanism
hLCN2-R-NTD retains high degrees of structural heterogeneity upon interacting with NGAL. hLCN2-R-NTD binds to NGAL via its relatively hydrophobic C-terminal part and, more surprising, that the formation of the complex leads to severe line broadening for both partners, probably due to the presence of conformational disorder in the complex.

Mechanism category
tethering

Significance
Fuzziness of the complex could be a mechanism that allows a fine-tuning of the interaction between NGAL and its cellular receptor or a biochemical mechanism allowing the receptor to discriminate between apo- and holo-NGAL.

Medical relevance
NGAL also has pleitropic functions by playing roles in organogenesis and cell differentiation, cell migration, apoptosis, and inflammation and is involved in cancer progression and metastasis.