Three-dimensional structure of an intact glycoside hydrolase family 15 glucoamylase from Hypocrea jecorina.

The three-dimensional structure of a complete Hypocrea jecorina glucoamylase has been determined at 1.8 A resolution. The presented structure model includes the catalytic and starch binding domains and traces the course of the 37-residue linker segment. While the structures of other fungal and yeast glucoamylase catalytic and starch binding domains have been determined separately, this is the first intact structure that allows visualization of the juxtaposition of the starch binding domain relative to the catalytic domain. The detailed interactions we see between the catalytic and starch binding domains are confirmed in a second independent structure determination of the enzyme in a second crystal form. This second structure model exhibits an identical conformation compared to the first structure model, which suggests that the H. jecorina glucoamylase structure we report is independent of crystal lattice contact restraints and represents the three-dimensional structure found in solution. The proposed starch binding regions for the starch binding domain are aligned with the catalytic domain in the three-dimensional structure in a manner that supports the hypothesis that the starch binding domain serves to target the glucoamylase at sites where the starch granular matrix is disrupted and where the enzyme might most effectively function.

Altered distribution and co-localization of polycystin-2 with polycystin-1 in MDCK cells after wounding stress.

Polycystin-1 and -2 are integral membrane glycoproteins defective in autosomal dominant polycystic kidney disease (ADPKD). Recent studies showed a coupled polycystin-1 and -2 action in cell signaling and channel activation suggesting an important biological role for the two proteins at the plasma membrane. To gain a better understanding about the (co)-distribution and dynamics of the polycystin-1 and -2 complex under stress conditions, we used a wound-healing model of Madine Darby canine kidney (MDCK) renal epithelial cells. In this model, cells near the wound edge undergo a process of reorganization to active migration, while cells further from the edge are unaffected and remain confluent. For the first time, endogenous polycystin-1 and -2 were found to partly co-localize in the plasma membrane of confluent monolayers, and both proteins co-localized in the primary cilium. Upon wound healing, the association of polycystin-2 to the membrane was greatly reduced at the wound edge and the submarginal cells. Polycystin-1 remained incorporated to the membrane at the edge of the cell sheet at all time points, although strongly reduced in lamellipodia-forming cells. Adherens junctions and desmosomes, and respective connected actin and keratin cytoskeleton were also disturbed in lamellipodia-forming cells. We propose that altered subcellular localization of polycystin-1 and -2 as a result of stress will affect signaling and other cellular processes mediated by these proteins.

Distinct subcellular expression of endogenous polycystin-2 in the plasma membrane and Golgi apparatus of MDCK cells.

Polycystin-2 is a predicted integral membrane protein with non-selective cation channel activity. The protein is encoded by the PKD2 gene, which is mutated in approximately 15% of patients with autosomal dominant polycystic kidney disease (ADPKD). Polycystin-2 can interact with the transmembrane protein polycystin-1, the product of the PKD1 gene. However, endoplasmic reticulum (ER) localization was reported for (heterologously expressed) polycystin-2 in cultured cells and baso-lateral localization has been reported in renal tissues. Using two polyclonal antisera raised against polycystin-2 we demonstrated distinct expression of the endogenous protein in the Golgi apparatus and the plasma membrane of MDCK cells. In contrast, most of the heterologously expressed polycystin-2 (PC2-EGFP) remained in the ER, substantially overlapping with the staining pattern of protein-disulfide isomerase (PDI), a marker for the ER. Only in a small subset of these cells weak plasma membrane signals were observed. Membrane staining was also suggested by immunoelectron microscopy and was confirmed by subcellular fractionation on sucrose density gradients. The plasma membrane staining disappeared following extraction with a buffer containing Triton X-100, whereas signals for polycystin-1 and E-cadherin remained visible, suggesting that polycystin-2 is neither tightly bound to the Triton X-100 insoluble cytoskeleton, nor to these proteins. We conclude that endogenous polycystin-2 is transported via the Golgi apparatus to the plasma membrane and has a broader membrane localization than polycystin-1. These data suggest that polycystin-2 can move freely in certain regions of the membrane where it probably functions as a channel, activated by, or in complex with, polycystin-1.