The cellular function of the cancer-associated RNA-binding protein La has been linked to translation of viral and cellular mRNAs. Recently, we have shown that the human La protein stimulates IRES-mediated translation of the cooperative oncogene CCND1 in cervical cancer cells. However, there is little known about the underlying molecular mechanism by which La stimulates CCND1 IRES-mediated translation, and we propose that its RNA chaperone activity is required. Herein, we show that La binds close to the CCND1 start codon and demonstrate that La's RNA chaperone activity can change the folding of its binding site. We map the RNA chaperone domain (RCD) within the C-terminal region of La in close proximity to a novel AKT phosphorylation site (T389). Phosphorylation at T389 by AKT-1 strongly impairs its RNA chaperone activity. Furthermore, we demonstrate that the RCD as well as T389 is required to stimulate CCND1 IRES-mediated translation in cells. In summary, we provide a model whereby a novel interplay between RNA-binding, RNA chaperoning and AKT phosphorylation of La protein regulates CCND1 IRES-mediated translation.
CEACAM1 is a cellular adhesion molecule whose protein expression is down-regulated in several carcinomas and which also contributes to the pathogenicity of Neisseria by acting as a receptor for Opa proteins. The crystal structure of the N-terminal (D1) domain of human CEACAM1 has been determined at 2.2 Angstrom resolution. The structure shows several differences compared with a lower resolution model of the same domain from mouse solved previously, especially in the functional regions. Mapping of the sites of mutations that lower or abolish the binding of CEACAM1 to Opa proteins shows a distinct clustering of residues on the GFCC'C'' face of the molecule. Prominent amongst these are residues in the C, C' and F strands and the CC' loop. A similar analysis shows that the region responsible for homophilic or heterophilic interactions of CEACAM1 is also on the GFCC'C'' face and overlaps partially with the Opa-binding region. This higher resolution structure of CEACAM1 will facilitate a more precise dissection of its functional regions in the context of neisserial pathogenesis, cellular adhesion and immune evasion.
The global incidence of the sexually transmitted disease gonorrhea is expected to rise due to the spread of Neisseria gonorrhoeae strains with decreased susceptibility to extended-spectrum cephalosporins (ESCs). ESC resistance is conferred by mosaic variants of penicillin-binding protein 2 (PBP2) that have diminished capacity to form acylated adducts with cephalosporins. To elucidate the molecular mechanisms of ESC resistance, we conducted a biochemical and high-resolution structural analysis of PBP2 variants derived from the decreased-susceptibility N. gonorrhoeae strain 35/02 and ESC-resistant strain H041. Our data reveal that mutations both lower affinity of PBP2 for ceftriaxone and restrict conformational changes that normally accompany acylation. Specifically, we observe that a G545S substitution hinders rotation of the β3 strand necessary to form the oxyanion hole for acylation and also traps ceftriaxone in a noncanonical configuration. In addition, F504L and N512Y substitutions appear to prevent bending of the β3–β4 loop that is required to contact the R1 group of ceftriaxone in the active site. Other mutations also appear to act by reducing flexibility in the protein. Overall, our findings reveal that restriction of protein dynamics in PBP2 underpins the ESC resistance of N. gonorrhoeae.
Resistance of Neisseria gonorrhoeae to expanded-spectrum cephalosporins such as ceftriaxone and cefixime has increased markedly in the past decade. The primary cephalosporin-resistance determinant is a mutated penA gene, which encodes the essential peptidoglycan transpeptidase, penicillin-binding protein 2 (PBP2). Decreased susceptibility and resistance can be conferred by mosaic penA alleles containing upwards of 60 amino acid changes relative to wild-type PBP2, or by non-mosaic alleles with relatively few mutations, the most important of which occurs at Ala501 located near the active site of PBP2. Recently, fully cefixime- and ceftriaxone-resistant clinical isolates were identified that harbored a mosaic penA allele with an A501P mutation. To examine the potential of mutations at Ala501 to increase resistance to expanded-spectrum cephalosporins, we randomized codon-501 in a mosaic penA allele and transformed N. gonorrhoeae to increased cefixime resistance. Interestingly, only five substitutions of Ala501 (A501V, A501T, A501P, A501R, A501S) were isolated that increased resistance and preserved essential transpeptidase function. To understand their structural implications, these mutations were introduced into the non-mosaic PBP2-6140CT, which contains four C-terminal mutations present in PBP2 from the penicillin-resistant strain FA6140. The crystal structure of PBP2-6140CT-A501T was solved and revealed ordering of a loop near the active site and a new hydrogen bond involving Thr501 that connects the loop and the SxxK conserved active-site motif. The structure suggests that increased rigidity in the active site region is a mechanism for cephalosporin resistance mediated by Ala501 mutations in PBP2.
Insertion of an aspartate residue at position 345a in penicillin-binding protein 2 (PBP 2), which lowers the rate of penicillin acylation by ~6-fold, is commonly observed in penicillin-resistant strains of Neisseria gonorrhoeae. Here, we show that insertions of other amino acids also lower the penicillin acylation rate of PBP 2, but none supported growth of N. gonorrhoeae, indicating loss of essential transpeptidase activity. The Asp345a mutation likely acts by altering the interaction between its adjacent residue, Asp346, in the β2a-β2d hairpin loop and Ser363, the middle residue of the SXN active site motif. Because the adjacent aspartate creates ambiguity in the position of the insertion, we also examined if insertions at position 346a could confer decreased susceptibility to penicillin. However, only aspartate insertions were identified, indicating that only an Asp-Asp couple can confer resistance and retain transpeptidase function. The importance of the Asp346-Ser363 interaction was assessed by mutation of each residue to Ala. Although both mutants lowered the acylation rate of penicillin G by 5-fold, neither could support growth of N. gonorrhoeae, again indicating loss of transpeptidase function. Interaction between a residue in the equivalent of the β2a-β2d hairpin loop and the middle residue of the SXN motif is observed in crystal structures of other Class B PBPs and its importance is also supported by multi-sequence alignments. Overall, these results suggest that this conserved interaction can be manipulated (e.g. by insertion) to lower the acylation rate by β-lactam antibiotics and increase resistance, but only if essential transpeptidase activity is preserved.
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