3D high- and super-resolution imaging using single-objective SPIM.

Single-objective selective-plane illumination microscopy (soSPIM) is achieved with micromirrored cavities combined with a laser beam-steering unit installed on a standard inverted microscope. The illumination and detection are done through the same objective. soSPIM can be used with standard sample preparations and features high background rejection and efficient photon collection, allowing for 3D single-molecule-based super-resolution imaging of whole cells or cell aggregates. Using larger mirrors enabled us to broaden the capabilities of our system to image Drosophila embryos.

Structural characterization of a eukaryotic chaperone--the ribosome-associated complex.

Ribosome-associated chaperones act in early folding events during protein synthesis. Structural information is available for prokaryotic chaperones (such as trigger factor), but structural understanding of these processes in eukaryotes lags far behind. Here we present structural analyses of the eukaryotic ribosome-associated complex (RAC) from Saccharomyces cerevisiae and Chaetomium thermophilum, consisting of heat-shock protein 70 (Hsp70) Ssz1 and the Hsp40 Zuo1. RAC is an elongated complex that crouches over the ribosomal tunnel exit and seems to be stabilized in a distinct conformation by expansion segment ES27. A unique α-helical domain in Zuo1 mediates ribosome interaction of RAC near the ribosomal proteins L22e and L31e and ribosomal RNA helix H59. The crystal structure of the Ssz1 ATPase domain bound to ATP-Mg²âº explains its catalytic inactivity and suggests that Ssz1 may act before the RAC-associated chaperone Ssb. Our study offers insights into the interplay between RAC, the ER membrane-integrated Hsp40-type protein ERj1 and the signal-recognition particle.

The metastasis-promoting phosphatase PRL-3 shows activity toward phosphoinositides.

Phosphatase of regenerating liver 3 (PRL-3) is suggested as a biomarker and therapeutic target in several cancers. It has a well-established causative role in cancer metastasis. However, little is known about its natural substrates, pathways, and biological functions, and only a few protein substrates have been suggested so far. To improve our understanding of the substrate specificity and molecular determinants of PRL-3 activity, the wild-type (WT) protein, two supposedly catalytically inactive mutants D72A and C104S, and the reported hyperactive mutant A111S were tested in vitro for substrate specificity and activity toward phosphopeptides and phosphoinositides (PIPs), their structural stability, and their ability to promote cell migration using stable HEK293 cell lines. We discovered that WT PRL-3 does not dephosphorylate the tested phosphopeptides in vitro. However, as shown by two complementary biochemical assays, PRL-3 is active toward the phosphoinositide PI(4,5)P(2). Our experimental results substantiated by molecular docking studies suggest that PRL-3 is a phosphatidylinositol 5-phosphatase. The C104S variant was shown to be not only catalytically inactive but also structurally destabilized and unable to promote cell migration, whereas WT PRL-3 promotes cell migration. The D72A mutant is structurally stable and does not dephosphorylate the unnatural substrate 3-O-methylfluorescein phosphate (OMFP). However, we observed residual in vitro activity of D72A against PI(4,5)P(2), and in accordance with this, it exhibits the same cellular phenotype as WT PRL-3. Our analysis of the A111S variant shows that the hyperactivity toward the unnatural OMFP substrate is not apparent in dephosphorylation assays with phosphoinositides: the mutant is completely inactive against PIPs. We observed significant structural destabilization of this variant. The cellular phenotype of this mutant equals that of the catalytically inactive C104S mutant. These results provide a possible explanation for the absence of the conserved Ser of the PTP catalytic motif in the PRL family. The correlation of the phosphatase activity toward PI(4,5)P(2) with the observed phenotypes for WT PRL-3 and the mutants suggests a link between the PI(4,5)P(2) dephosphorylation by PRL-3 and its role in cell migration.