The extracellular-region annotation on this horse record is a vocabulary mapping from the UniProt subcellular-location keyword SL-0243, not an equine observation; it inherits whatever the sequence-based location assignment asserts and so cannot itself settle whether this protein form is secreted.

Photomedin-1 (OLFML2A) has a defined domain architecture with tandem CXCXCX9C motifs in the N-terminal region, a coiled-coil domain, and a C-terminal olfactomedin domain.

Photomedin-1 is secreted as a disulphide-bonded dimer carrying O-linked carbohydrate chains and is proteolytically processed mid-molecule after secretion, so secretory entry is a prerequisite for the characterized mature form rather than an incidental detail.

Among extracellular matrix components tested, the photomedins bound preferentially to chondroitin sulphate-E and heparin - a binding preference, not a demonstration of matrix assembly or of a signaling receptor.

In the retina, photomedin-1 is selectively expressed in the photoreceptor outer segment, distinguishing it from photomedin-2/OLFML2B, which is expressed in all retinal neurons.

OLFML2A is enriched in the glomerulus, establishing a kidney protein pool distinct from the retinal one.

Within the glomerulus OLFML2A is expressed exclusively by podocytes and localizes to their major processes, verified by four independent methods including immunoelectron microscopy.

The screen that placed OLFML2A among extracellular matrix proteins assayed candidates for matrix-assembling activity, matrix interactions, glycosaminoglycan modification and cell adhesion, so which of those a given hit satisfied has to be traced per gene before a matrix-organization claim can be made for it.

The horse OLFML2A entry is an unreviewed 599-residue TrEMBL record whose protein name derives solely from an Ensembl gene model, with no equine experimental characterization.

The record carries an olfactomedin-like domain signature, which is what secures family identity independently of the missing N-terminal features.

The selected horse record carries the Secreted keyword, but it is assigned by an ARBA family rule rather than by a signal-peptide feature on this sequence.

The horse sequence is a close counterpart of human OLFML2A over nearly its whole length (91.3% identity, 598 paired residues, 99.8% horse coverage), so the differences are localized rather than global.

The alignment places a 26-residue gap opposite the human signal peptide and a further 22-residue internal deletion opposite human residues 123-144; the latter ends upstream of the predicted human coiled-coil at residues 157-183.

The analysis explicitly declines to conclude that the horse gene product is intracellular, or that the prediction pipeline was fed a leaderless sequence - the discrepancy is recorded as a model limitation, not a demonstrated biological difference.

Human OLFML2A is annotated as secreted, and the annotation is itself a similarity transfer from the mouse protein rather than a direct human observation.

Secretion depends on a signal peptide occupying human residues 1-27, precisely the region for which the horse model has no counterpart.

The human protein is a homodimer that binds heparin and chondroitin sulfate E.

Human UniProt annotates residues 28-652 as the mature post-cleavage OLFML2A chain.

The olfactomedin-like domain spans human residues 394-652, i.e. the conserved C-terminal two-thirds that the horse model does retain.

Human OLFML2A also localizes to podocyte major processes and colocalizes with vimentin through podocyte development, a kidney pool separate from the retinal expression.

The human gene has three annotated splice isoforms, which is a further reason a single divergent N-terminus in another species is better treated as a transcript-model question than as evidence of lost secretion.

The horse OLFML2A locus encodes a leader-bearing 650-residue RefSeq product (XP_023484611.2) that matches human canonical isoform 1. The benchmark A0A9L0SKW1 is one of several leaderless alternative models, and it additionally lacks the segment opposite human 123-144.