The hsp90 1 family of molecular chaperones is required for the activation of a large number of proteins, including those involved in eukaryotic cell cycle regulation, signal transduction, immune response, and transcription (1, 2). GRP94 (also known as gp96), the endoplasmic reticulum paralog of cytoplasmic Hsp90, participates in the maturation of proteins that are transported to the cell surface, including Toll-like receptors and certain integrins (3), or secreted, such as IgGs (4, 5). The activity of the hsp90 family chaperones is regulated by ATP (6). Fungally derived ansamycin antibiotics such as geldanamycin and radicicol (7) compete for the ATP binding site (8 -11) and exhibit potent anti-tumor activity by acting as pan-hsp90 inhibitors (12-17). Given their central importance in the biology of protein folding and in cellular stress response, and their potential as targets in a variety of therapeutic strategies, the mechanism by which hsp90 chaperone activity is regulated and the chemistry of their interactions with client substrates are under intensive investigation.GRP94 and Hsp90 exist as obligate homodimers, with each subunit consisting of an N-terminal regulatory and ligand binding domain, a charged region, a middle domain, and a C-terminal dimerization domain. This organization and the structural similarity of their N-terminal ATP binding domains characterize the GHKL family of proteins (18), whose members in addition to Hsp90s also include DNA gyrase, histidine kinases, DNA topoisomerase II, and MutL. By analogy to DNA gyrase (19) and MutL (20), it has been proposed that Hsp90 chaperone activity is linked to an N-domain conformational change that is coupled to ATP binding and hydrolysis and that results in the dimerization of the N-domains (21, 22). Mechanistic explanations of how ATP binding and hydrolysis are coupled to Hsp90 activity have been hindered, however, by the lack of any observed change in the conformation of the Ndomain in response to ligands. The complete protein, rather than just the isolated N-domain, is required for ATP hydrolysis (6, 23), suggesting that conformational rearrangements resulting in a close interaction between the N-and middle domains are important for Hsp90 activity. Interactions with co-chaperones may also be important catalysts for ATP hydrolysis in human Hsp90 as well (24).GRP94 and Hsp90 are greater than 50% identical in their N-domains, yet the biochemical hallmarks of Hsp90 chaperone activity are largely missing for this paralog. ATP hydrolysis above background levels has not been detected for GRP94 (25), nor have co-chaperones or partner proteins been identified. In addition, although Hsp90 binds ADP/ATP with micromolar affinity (11), the binding constant for adenosine nucleotides to GRP94 is estimated to be of several millimolar units (25). These differences suggest that the structure of GRP94 may reveal an alternative regulatory mechanism for GRP94. The structure of the N-domain of GRP94 in complex with a series of inhibitory ligands was reported recently and...

SUMMARY Background Renal failure occurs in 5–18% of sickle cell disease (SCD) patients and is associated with early mortality. At risk SCD patients cannot be identified prior to the appearance of proteinuria and the pathobiology is not well understood. The MYH9 and APOL1 genes have been associated with risk for focal segmental glomerulosclerosis and end-stage renal disease in African Americans. Methods We genotyped 26 SNPs in MYH9 and 2 SNPs in APOL1 in 521 unrelated adult (18–83 years) SCD patients screened for proteinuria. Using logistic regression, SNPs were evaluated for association with proteinuria. Results Eight MYH9 SNPs and one APOL1 SNP were nominally associated with proteinuria. Six SNPs remained significant after multiple testing correction (p < 0.0025), and a risk haplotype was associated with proteinuria (p=0.001). Using multiple regression, association with APOL1 diminished in the presence of MYH9 SNPs. Glomerular filtration rate was negatively correlated with proteinuria (p < 0.0001), and was nominally associated with MYH9 and APOL1 in age-adjusted analyses. Conclusion Our data provide insight into the pathobiology of renal dysfunction in SCD, suggesting that MYH9 is more strongly associated than APOL1. These data also provide the opportunity for early identification of patients at risk and new therapeutics.