The transthyretin-related protein family.

A number of proteins related to the homotetrameric transport protein transthyretin (TTR) forms a highly conserved protein family, which we present in an integrated analysis of data from different sources combined with an initial biochemical characterization. Homologues of the transthyretin-related protein (TRP) can be found in a wide range of species including bacteria, plants and animals, whereas transthyretins have so far only been identified in vertebrates. A multiple sequence alignment of 49 TRP sequences from 47 species to TTR suggests that the tertiary and quaternary features of the three-dimensional structure are most likely preserved. Interestingly, while some of the TRP orthologues show as little as 30% identity, the residues at the putative ligand-binding site are almost entirely conserved. RT/PCR analysis in Caenorhabditis elegans confirms that one TRP gene is transcribed, spliced and predominantly expressed in the worm, which suggests that at least one of the two C. elegans TRP genes encodes a functional protein. We used double-stranded RNA-mediated interference techniques in order to determine the loss-of-function phenotype for the two TRP genes in C. elegans but detected no apparent phenotype. The cloning and initial characterization of purified TRP from Escherichia coli reveals that, while still forming a homotetramer, this protein does not recognize thyroid hormones that are the natural ligands of TTR. The ligand for TRP is not known; however, genomic data support a functional role involving purine catabolism especially linked to urate oxidase (uricase) activity.