期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2014
卷号：111
期号：44
页码：15746-15751
DOI：10.1073/pnas.1413018111
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceStreptococcus mutans adhesin P1 is a target of protective immunity and a vaccine candidate. P1's complex structure dictates its function and makes it of interest from a protein folding perspective as well. An interaction between N- and C-terminal sequences contributes to antigenicity, adherence behavior, and stability. This is now explained by the identification of a previously unidentified fold in which the N terminus forms a stabilizing scaffold at the base of P1's helical stalk to physically lock it in place via interactions with the C terminus. Disruption of this intramolecular lock not only negatively affects stability, but also prevents proper folding of the purified full-length protein. The cariogenic bacterium Streptococcus mutans uses adhesin P1 to adhere to tooth surfaces, extracellular matrix components, and other bacteria. A composite model of P1 based on partial crystal structures revealed an unusual complex architecture in which the protein forms an elongated hybrid alpha/polyproline type II helical stalk by folding back on itself to display a globular head at the apex and a globular C-terminal region at the base. The structure of P1's N terminus and the nature of its critical interaction with the C-terminal region remained unknown, however. We have cocrystallized a stable complex of recombinant N- and C-terminal fragments and here describe a previously unidentified topological fold in which these widely discontinuous domains are intimately associated. The structure reveals that the N terminus forms a stabilizing scaffold by wrapping behind the base of P1's elongated stalk and physically "locking" it into place. The structure is stabilized through a highly favorable {Delta}Gsolvation on complex formation, along with extensive hydrogen bonding. We confirm the functional relevance of this intramolecular interaction using differential scanning calorimetry and circular dichroism to show that disruption of the proper spacing of residues 989-1001 impedes folding and diminishes stability of the full-length molecule, including the stalk. Our findings clarify previously unexplained functional and antigenic properties of P1.
