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Nucleotide sequence analysis reveals novel features of the phase-variable cytadherence accessory protein HMW3 of
K F Ogle1, K K Lee, D C Krause
1Department of Microbiology, University of Georgia, Athens 30602.
Abstract:
The Mycoplasma pneumoniae hmw3 gene was sequenced and its gene product was characterized with the goal of elucidating the functional role of HMW3 as an accessory component in cytadherence. A total of 2,016 bp of the hmw3 locus was sequenced, revealing an open reading frame large enough to encode a 672-amino-acid protein with a molecular weight of 73,725. No consensuslike ribosome binding or promoter sequences were identified. However, hmw3 was flanked by upstream and downstream open reading frames. Gene identity was confirmed by comparing the deduced amino acid sequence with the amino acid sequence obtained directly from N-terminal amino acid sequencing of HMW3. Analysis of the deduced amino acid sequence indicated an acidic pI (4.4), an unusual distribution of charged residues, a high degree of hydrophilicity, and a high proline content for HMW3. M. pneumoniae protein profiles obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) following culturing in the presence of [3H]Pro were consistent with the high Pro content predicted for HMW3 and indicated the same for cytadherence accessory protein HMW1. A discrepancy existed between the predicted size of HMW3 (Mr, 73,725) and the size of HMW3 obtained by SDS-PAGE (Mr, 140,000). Variable electrophoretic mobility in SDS-PAGE was observed at different acrylamide concentrations for HMW3, indicating that anomalous migration might account for the size discrepancy. The relative mobility of HMW3 was enhanced only slightly in the presence of magnesium acetate, suggesting that the unusual charge distribution might only be partially responsible for the anomalous migration. Secondary structure predictions were dominated by beta-sheets and nonrepetitive turns or coils, suggesting a probable extended, rigid conformation for HMW3. Finally, an unusual, highly acidic domain (residues 180 to 280) which might have particular functional significance relative to the role of HMW3 as a cytadherence accessory protein was identified.
Insights
The Mycoplasma pneumoniae HMW3 protein
Area of Science:
- Microbiology
- Molecular Biology
- Protein Biochemistry
Background:
- Mycoplasma pneumoniae is a significant human respiratory pathogen.
- Cytadherence is a crucial step in M. pneumoniae pathogenesis.
- HMW proteins are known to be involved in cytadherence.
Purpose of the Study:
- To characterize the HMW3 protein from Mycoplasma pneumoniae.
- To elucidate the functional role of HMW3 in cytadherence.
Main Methods:
- Gene sequencing of the hmw3 locus.
- N-terminal amino acid sequencing.
- Protein profiling using SDS-PAGE.
- Secondary structure prediction.
Main Results:
- Sequencing revealed a 672-amino acid protein (HMW3) with a predicted molecular weight of 73,725 Da.
- HMW3 exhibits an acidic pI, high hydrophilicity, high proline content, and an unusual acidic domain.
- SDS-PAGE showed a discrepancy in molecular weight (140,000 Da), attributed to anomalous electrophoretic mobility.
- Secondary structure predictions suggest an extended, rigid conformation.
Conclusions:
- HMW3 is a novel accessory protein involved in M. pneumoniae cytadherence.
- The protein's unique biochemical properties, including an acidic domain, may be critical for its function.
- Anomalous migration in SDS-PAGE requires careful consideration during protein analysis.