Structural and Functional Similarity between Yersinia pestis Capsular Protein Caf1 and Human Interleukin-1β

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Abstract
A comparative study of the structural and functional properties of recombinant Yersinia pestis Caf1 and human IL-1β was performed. According to Fourier transform infrared spectroscopy (FTIR) and circular dichroism (CD) data, IL-1β and Caf1 are typical β-structural proteins. Neither protein interacts with the hydrophobic probe ANS (8-anilino-1-naphthalenesulfonate) under physiological conditions. Specific binding of Caf1 [Kd = (5.4 ± 0.1) × 10-10 M] to interleukin-1 receptors (IL-1Rs) on the surface of finite mouse fibroblasts (line NIH 3T3) was observed. Caf1 is able to inhibit high-affinity binding of 125I-labeled IL-1β to NIH 3T3 cells, and in the presence of Caf1, the binding of [125I]IL-1β is characterized by a Kd of (2.0 ± 0.3) × 10-9 M. Caf1 binding to IL-1R could reflect adhesive properties of the capsular subunits responsible for the contact of bacteria with the host immunocompetent cells. In its turn, this may represent a signal for the initiation of the expression and secretion of the proteins of Y. pestis Yop virulon. Thus, these results help to explain the importance of Caf1 in the interaction of Y. pestis with the host immune system.