title: When a module is not a domain: the case of the REJ module and the redefinition of the architecture of polycystin-1.
abstract: The extracellular region of a group of cell-surface receptors known as the polycystic kidney disease 1 family, containing, among others, polycystin-1, has been controversially described as containing four FNIII (fibronectin type III) domains or one REJ (receptor of egg jelly protein) module in the same portion of polypeptide. Stimulated by recent atomic force microscopy work, we re-examined the similarity of these four domains with a FNIII sequence profile showing the evolutionary relationship. Two of the predicted domains could be expressed in bacteria and refolded to give a protein suitable for biophysical study, and one of these expressed solubly. CD spectroscopy showed that both domains contain a significant amount of β-sheet, in good agreement with theoretical predictions. Confirmation of independent folding as a domain is obtained from highly co-operative thermal and urea unfolding curves. Excellent dispersion of peaks in the high-field region of one-dimensional NMR spectra confirms the presence of a hydrophobic core. Analytical ultracentrifugation and analytical gel filtration agree very well with the narrow linewidths in the NMR spectra that at least one of the domains is monomeric. On the basis of this combined theoretical and experimental analysis, we show that the extracellular portion of polycystin-1 does indeed contain β-sheet domains, probably FNIII, and that, consequently, the REJ module is not a single domain.
authors.affiliation: Department of Biochemistry, University of Leicester, Leicester LE1 9HN, UK.
authors.forename: Samantha
authors.initials: S
authors.lastname: Schröder
authors.forename: Franca
authors.initials: F
authors.lastname: Fraternali
authors.forename: Xueping
authors.initials: X
authors.lastname: Quan
authors.forename: David
authors.initials: D
authors.lastname: Scott
authors.forename: Feng
authors.initials: F
authors.lastname: Qian
authors.forename: Mark
authors.initials: M
authors.lastname: Pfuhl
mesh_terms: D000595:Amino Acid Sequence; D002942:Circular Dichroism; D005786:Gene Expression Regulation / Q000502:physiology*; D006801:Humans; D008969:Molecular Sequence Data; D009154:Mutation; D011489:Protein Denaturation; D058849:Protein Refolding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins; D050396:TRPP Cation Channels / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D013696:Temperature
chemical_list: D011994:Recombinant Proteins; D050396:TRPP Cation Channels