Polycystin-1 is a large transmembrane protein, which, when mutated, causes autosomal dominant polycystic kidney disease, one of the most common life-threatening genetic diseases that is a leading cause of kidney failure. The REJ (receptor for egg lelly) module is a major component of PC1 ectodomain that extends to about 1000 amino acids. Many missense disease-causing mutations map to this module; however, very little is known about the structure or function of this region. We used a combination of homology molecular modeling, protein engineering, steered molecular dynamics (SMD) simulations, and single-molecule force spectroscopy (SMFS) to analyze the conformation and mechanical stability of the first ~420 amino acids of REJ. Homology molecular modeling analysis revealed that this region may contain structural elements that have an FNIII-like structure, which we named REJd1, REJd2, REJd3, and REJd4. We found that REJd1 has a higher mechanical stability than REJd2 (~190 pN and 60 pN, resp.). Our data suggest that the putative domains REJd3 and REJd4 likely do not form mechanically stable folds. Our experimental approach opens a new way to systematically study the effects of disease-causing mutations on the structure and mechanical properties of the REJ module of PC1. 1. Introduction PC1 is a large transmembrane protein, which, when mutated, causes autosomal dominant polycystic kidney disease (ADPKD), one of the most common life-threatening genetic diseases that is a leading cause of kidney failure [1]. PC1 may have a role in sensing of flow [2, 3], pressure [4] and the regulation of the cell cycle [5] and cell polarity [6]. PC1 may sense signals from the primary cilia, neighboring cells, and extracellular matrix and transduces them into cellular responses that regulate proliferation, adhesion, and differentiation that are essential for the control of renal tubules and kidney morphogenesis [1, 3, 7, 8]. The predicted amino acid sequence of PC1 (Figure 1(a)) suggests that it is a large multidomain membrane protein with 11 transmembrane domains. Its N-terminal extracellular region contains 4 leucine-rich repeats ((LRR) 250 amino acid long), a C-type lectin domain ((CLD) 130 amino acid long), a low-density-lipoprotein-like domain (LDL-A domain), 16 Ig-like domains (PKD domains, each 90 amino acid) and a region that is homologous to a sea urchin protein called receptor for egg jelly (REJ) [9, 10]. The PKD domains in PC1 have a similar topology fibronectin type III (FNIII) domain found in other modular proteins with structural and mechanical roles (recently
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