The HYR (HYalin Repeat) domain is an extracellular domain of about 80-100 amino acids. It has been named after the hyalin protein, which is composed exclusively of repeats of this domain. The HYR domain is found in several eukaryotic proteins, either in multiple copies as in hyalin or in association with other domains like the CCP (sushi) domain, the Von Willebrand factor type A (VWA) domain (see <PDOC50234>), the EGF-like domain (see <PDOC00021>), the calcium-binding EGF-like domain (see <PDOC00913>), the pentraxin domain (see <PDOC00261>), the CUB domain (see <PDOC00908>), the LDL-receptor class A domain (see <PDOC00929>), the C-type lectin domain (see <PDOC00537>) or the discoidin domain. As the HYR domains of hyalin have been shown to contain the ligand for the hyalin cell surface receptor, the HYR domain can also be expected to play a direct role in cellular adhesion in other proteins in which it is present [1].

Secondary structure predictions of the HYR domain indicate an all-β fold including seven β-strands. The HYR domain shares clear sequence similarities limited to the C-terminal region with the Fn3 and PKD domains and is believed to belong to the immunoglobulin-like fold [1].

Some proteins containing a HYR domain are listed below:

• Echinoderm hyalin, a protein of the extra-embryonic matrix.
• Mouse polydom, a secreted protein with pentraxin, CCP, EGF and VWA domains [2].
• Mammalian sushi-repeat-containing protein (SRPX). It is thought to be located at the photoreceptor cell surface. In human, the gene encoding SRPX is deleted in patients with X-linked retinitis pigmentosa.
• Caenorhabditis elegans hypothetical proteins F55H12.3, F47C12.10 and W02C12.1.

The profile we developed covers the entire HYR domain.