Human LRIT3 (Q3SXY7) is a reviewed 679-residue type-I single-pass membrane
glycoprotein. UniProt maps a signal peptide at 1-19, an extracellular region with
five LRRs, an LRR C-terminal cap, an Ig-like domain, and an FN3 domain, a
transmembrane helix at 583-603, and a cytoplasmic tail. This architecture supports
an extracellular organizer role but does not itself establish adhesion, receptor,
or enzymatic activity [file:human/LRIT3/LRIT3-uniprot.txt].
Two human isoforms are curated. Isoform 2 lacks canonical residues 46-183 and
replaces residues 184-196, removing the annotated five-LRR repeat region while
retaining the later Ig-like, FN3, transmembrane, and cytoplasmic regions by sequence.
The isoform-2 cDNA came from cerebellum, but no direct isoform-specific localization,
partner, retinal function, or disease effect has been established.
Human genetics establishes LRIT3 as a cause of autosomal-recessive complete
congenital stationary night blindness. The discovery study identified compound
heterozygous and additional biallelic truncating/missense alleles and reported that
"Human LRIT3 antibody staining revealed in the outer plexiform layer of the human"
retina a punctate pattern resembling bipolar-cell dendritic tips
PMID:23246293. This is direct human disease and localization evidence, but the
apparent postsynaptic position from antibody apposition must be reconciled with
subsequent mouse endogenous-tagging and rescue studies that place LRIT3 presynaptically
in photoreceptors.
Mouse loss-of-function studies show that LRIT3 is required for ON-pathway retinal
signaling and organization of the depolarizing-bipolar-cell glutamate signalplex.
One study concludes that LRIT3 is involved in "coordination of the transsynaptic communication between cones and ON-BCs during synapse formation and function"
PMID:28334377. Another demonstrated presynaptic rod expression and rescue, stating
that LRIT3 acts "as a transsynaptic organizer of the postsynaptic complex required for normal"
synaptic function PMID:31189098.
LRIT3 physically associates with nyctalopin in a heterologous co-immunoprecipitation
assay and is required for nyctalopin and downstream TRPM1 localization in mouse
retina: "LRIT3 interacts with and is required for expression of nyctalopin, and thus"
TRPM1 at depolarizing bipolar-cell dendritic tips PMID:31959619. This supports an
organizer mechanism and a partner-specific binding hypothesis, but not an obligate
stable complex in every retinal synapse.
Endogenous SMART tagging in mouse retina further supports a photoreceptor origin:
"LRIT3 is confined to pre-synaptic compartment" PMID:40263339. The study also detected tagged
LRIT3 in cone pedicles. The review therefore treats dendritic-tip/perikaryon terms as
historical localization descriptions that require careful cell-side interpretation,
not proof that LRIT3 is synthesized or functions postsynaptically.
The 2026 domain-deletion rescue study reports that "The IG domain is required for the localization of TRPM1 to the"
signalplex and that the LRR domain is needed for key
trafficking/assembly functions PMID:42055330. The proposed direct LRR-nyctalopin and
Ig-TRPM1 interfaces remain models rather than purified binary-interface
demonstrations.
The original human LRIT3 study used overexpressed proteins in cultured cells and
reported that LRIT3 influences FGFR1 maturation and signaling. Its discussion states
that "our results strongly suggest that LRIT3 facilitates ER export of FGFR1" while
also considering receptor overexpression and growth factors in serum as alternatives
PMID:22673519. This supports the existing experimental regulation-of-FGFR-signaling
annotation as a real assay result, but it is kept outside the retinal core and is not
generalized to an endogenous human developmental pathway or autonomous LRIT3 receptor
activity.