In Saccharomyces cerevisiae, ASH1 mRNA is localized to the tip of daughter cells during anaphase of the cell cycle. ASH1 mRNA localization is dependent on four cis-acting localization elements as well as Myo4p, She2p, and She3p. Myo4p, She2p, and She3p are hypothesized to form a heterotrimeric protein complex that directly transports ASH1 mRNA to daughter cells. She2p is an RNA-binding protein that directly interacts with ASH1 cis-acting localization elements and associates with She3p. Here we report the identification of seven She2p mutants-N36S, R43A, R44A, R52A, R52K, R63A, and R63K-that result in the delocalization of ASH1 mRNA. These mutants are defective for RNA-binding activity but retain the ability to interact with She3p, indicating that a functional She2p RNA-binding domain is not a prerequisite for association with She3p. Furthermore, the nuclear/cytoplasmic distribution for the N36S and R63K She2p mutants is not altered, indicating that nuclear/cytoplasmic trafficking of She2p is independent of RNA-binding activity. Using the N36S and R63K She2p mutants, we observed that in the absence of She2p RNA-binding activity, neither Myo4p nor She3p is asymmetrically sorted to daughter cells. However, in the absence of She2p, Myo4p and She3p can be asymmetrically segregated to daughter cells by artificially tethering mRNA to She3p, implying that the transport and/or anchoring of the Myo4p/She3p complex is dependent on the presence of associated mRNA.
Ail is a 17-kDa chromosomally encoded outer membrane protein that mediates serum resistance (complement resistance) in the pathogenic Yersiniae (Yersinia pestis, Y. enterocolitica, and Y. pseudotuberculosis). In this article, we demonstrate that Y. pseudotuberculosis Ail from strains PB1, 2812/79, and YPIII/pIB1 (serotypes O:1a, O:1b, and O:3, respectively) can bind the inhibitor of the classical and lectin pathways of complement, C4b-binding protein (C4BP). Binding was observed irrespective of serotype tested and independently of YadA, which is the primary C4BP receptor of Y. enterocolitica. Disruption of the ail gene in Y. pseudotuberculosis resulted in loss of C4BP binding. Cofactor assays revealed that bound C4BP is functional, because bound C4BP in the presence of factor I cleaved C4b. In the absence of YadA, Ail conferred serum resistance to strains PB1 and YPIII, whereas serum resistance was observed in strain 2812/79 in the absence of both YadA and Ail, suggesting additional serum resistance factors. Ail from strain YPIII/pIB1 alone can mediate serum resistance and C4BP binding, because its expression in a serum-sensitive laboratory strain of Escherichia coli conferred both of these phenotypes. Using a panel of C4BP mutants, each deficient in a single complement control protein domain, we observed that complement control protein domains 6–8 are important for binding to Ail. Binding of C4BP was unaffected by increasing heparin or salt concentrations, suggesting primarily nonionic interactions. These results indicate that Y. pseudotuberculosis Ail recruits C4BP in a functional manner, facilitating resistance to attack from complement.
We have previously shown that C3 binding to serum-resistant nontypeable Haemophilus influenzae (NTHi) strain R2866 is slower than C3 binding to a serum-sensitive strain. Ab-dependent classical pathway activation is required for complement-dependent killing of NTHi. To further characterize the mechanism(s) of serum resistance of R2866, we compared binding of complement component C4b to R2866 with a serum-sensitive variant, R3392. We show that C4b binding to R2866 relative to R3392 was delayed, suggesting regulation of the classical pathway of complement. Increased C4b deposition on R3392 was independent of the amount and subclass of Ab binding, suggesting that an impediment to C4b binding existed on R2866. Immunoblotting and mass spectrometry indicated that lipooligosaccharide and outer membrane proteins P2 and P5 were targets for C4b. P2 and P5 sequences and expression levels were similar in both strains. Insertional inactivation of the phase-variable lipooligosaccharide biosynthesis gene lgtC in R2866 augmented C4b deposition to levels seen with R3392 and rendered the bacteria sensitive to serum and whole blood. These results suggest a direct role of lgtC expression in the inhibition of C4b deposition and consequent serum resistance of R2866. Alteration of surface glycans of NTHi may be a critical event in determining the ability of a strain to evade host defenses and cause disseminated infection.
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