Method for dialysis of samples in microliter volumes

NMR structure of the 5' splice site in the group IIB intron Sc.ai5γ--conformational requirements for exon-intron recognition

A crucial step of the self-splicing reaction of group II intron ribozymes is the recognition of the 5' exon by the intron. This recognition is achieved by two regions in domain 1 of the intron, the exon-binding sites EBS1 and EBS2 forming base pairs with the intron-binding sites IBS1 and IBS2 located at the end of the 5' exon. The complementarity of the EBS1•IBS1 contact is most important for ensuring site-specific cleavage of the phosphodiester bond between the 5' exon and the intron. Here, we present the NMR solution structures of the d3' hairpin including EBS1 free in solution and bound to the IBS1 7-mer. In the unbound state, EBS1 is part of a flexible 11-nucleotide (nt) loop. Binding of IBS1 restructures and freezes the entire loop region. Mg(2+) ions are bound near the termini of the EBS1•IBS1 helix, stabilizing the interaction. Formation of the 7-bp EBS1•IBS1 helix within a loop of only 11 nt forces the loop backbone to form a sharp turn opposite of the splice site, thereby presenting the scissile phosphate in a position that is structurally unique.

Oligomerization and transglutaminase cross-linking of the cystatin CRES in the mouse epididymal lumen: potential mechanism of extracellular quality control

CRES (cystatin-related epididymal spermatogenic), a member of the cystatin superfamily of cysteine protease inhibitors, is expressed in the epididymis and spermatozoa, suggesting specialized roles in reproduction. Several cystatin family members oligomerize, including cystatin C that forms amyloid deposits associated with cerebral amyloid angiopathy. Our studies demonstrate that CRES also forms oligomers. Size exclusion chromatography revealed the presence of multiple forms of CRES in the epididymal luminal fluid, including SDS-sensitive and SDS-resistant high molecular mass complexes. In vitro experiments demonstrated that CRES is a substrate for transglutaminase and that an endogenous transglutaminase activity in the epididymal lumen catalyzed the formation of SDS-resistant CRES complexes. The use of a conformation-dependent antibody that recognizes only the oligomeric precursors to amyloid, negative stain electron microscopy, and Congo Red staining showed that CRES adopted similar oligomeric and fibrillar structures during its aggregation as other amyloidogenic proteins, suggesting that CRES has the potential to form amyloid in the epididymal lumen. The addition of transglutaminase, however, prevented the formation of CRES oligomers recognized by the conformation antibody by cross-linking CRES into an amorphous structure. We propose that transglutaminase activity in the epididymal lumen may function as a mechanism of extracellular quality control by diverting proteins such as CRES from the amyloidogenic pathway.