The 10 sea urchin receptor for egg jelly proteins (SpREJ) are members of the polycystic kidney disease-1 (PKD1) family

Abstract

Background: Mutations in the human polycystic kidney disease-1 (hPKD1) gene result in ~85% of cases of autosomal dominant polycystic kidney disease, the most frequent human monogenic disease. PKD1 proteins are large multidomain proteins involved in a variety of signal transduction mechanisms. Obtaining more information about members of the PKD1 family will help to clarify their functions. Humans have five hPKD1 proteins, whereas sea urchins have 10. The PKD1 proteins of the sea urchin, Strongylocentrotus purpuratus, are referred to as the Receptor for Egg Jelly, or SpREJ proteins. The SpREJ proteins form a subfamily within the PKD1 family. They frequently contain C-type lectin domains, PKD repeats, a REJ domain, a GPS domain, a PLAT/LH2 domain, 1-11 transmembrane segments and a C-terminal coiled-coil domain.

Results: The 10 full-length SpREJ cDNA sequences were determined. The secondary structures of their deduced proteins were predicted and compared to the five human hPKD1 proteins. The genomic structures of the 10 SpREJs show low similarity to each other. All 10 SpREJs are transcribed in either embryos or adult tissues. SpREJs show distinct patterns of expression during embryogenesis. Adult tissues show tissue-specific patterns of SpREJ expression.

Conclusion: Possession of a REJ domain of about 600 residues defines this family. Except for SpREJ1 and 3, that are thought to be associated with the sperm acrosome reaction, the functions of the other SpREJ proteins remain unknown. The sea urchin genome is one-fourth the size of the human genome, but sea urchins have 10 SpREJ proteins, whereas humans have five. Determination of the tissue specific function of each of these proteins will be of interest to those studying echinoderm development. Sea urchins are basal deuterostomes, the line of evolution leading to the vertebrates. The study of individual PKD1 proteins will increase our knowledge of the importance of this gene family.

Figure 1

The PKD1 family of proteins as exemplified by the 10 SpREJs and five hPKD1 protein architectures. The predicted secondary structures are shown. Domain boundaries were taken from the Pfam database. The REJ domain is split into two sections in SpREJ7 and partial REJ domains occur in SpREJ8 and 9, and hPKD1L2. hPKD1L3 does not contain a REJ domain. Most of these predicted structures show a GPS domain upstream of transmembrane helix 1 (TM1) and a PLAT/LH2 domain immediately after the first TM.

Figure 2

Expression of SpREJ transcripts during embryogenesis. Transcripts of SpREJ1 and 6 could not be detected in embryos. Transcript copy numbers per 100 ng of starting total RNA at each developmental stage for 8 SpREJs were first calculated. Transcript copy numbers were calculated from a standard curve generated in the same run and were normalized to the level of ubiquitin (SpUbi) transcripts at each developmental time. Final numbers are the averages of three replicas. There are about 50,000 transcripts per 100 ng total RNA of SpREJ8 at 22 hours of development. The standard error of each data point is shown (n = 3). Horizontal axes are hours of development at 16°C.

Figure 3

SpREJ transcript levels in six adult tissues. Transcript copy numbers per 100 ng of total RNA from each tissue was first calculated for all SpREJs. Transcript copy numbers were calculated from a standard curve generated in the same run and were normalized to the level of ubiquitin transcripts. It should be noted that the constancy of ubiquitin transcripts in adult tissues has not been determined. Final copy numbers were taken as the averages of duplicates. T, testis; O, ovary; M/T, muscle/test of body wall; G, gut; L, lantern (jaws, ligaments and muscles); C, coelomocytes (immunocytes).