An alpha-Helical Core Encodes the Dual Functions of the Chlamydial Protein IncA [Membrane Biology]

October 16th, 2014 by Ronzone, E., Wesolowski, J., Bauler, L. D., Hackstadt, T., Paumet, F.

Chlamydia is an intracellular bacterium that establishes residence within parasitophorous compartments (inclusions) inside host cells. Chlamydial inclusions are uncoupled from the endolysosomal pathway and undergo fusion with cellular organelles and with each other. To do so, Chlamydia expresses its own proteins on the surface of the inclusion using a Type III Secretion system. These proteins, termed Incs, are located at the interface of host and pathogen and carry out the functions necessary for Chlamydia survival. Among these Incs, IncA plays a critical role in both protecting the inclusion from lysosomal fusion and in inducing the homotypic fusion of inclusions. Within IncA are two regions homologous to eukaryotic SNARE (soluble N- ethylmaleimide sensitive factor attachment receptor) domains referred to as SNARE-like Domain 1 (SLD1) and SNARE-like Domain 2 (SLD2). We have previously demonstrated that although either SLD is sufficient to block endosomal SNARE-mediated membrane fusion, both domains are required for homotypic fusion. Here, we have discovered the functional core of IncA that retains the ability of wildtype IncA to both inhibit SNARE-mediated liposome fusion and promote homotypic fusion. Circular dichroism and analytical ultracentrifugation experiments show that this core region is composed almost entirely of α-helices and assembles into stable homodimers in solution. Altogether, we propose that both IncA functions are encoded in a structured core domain and that oligomerization of IncA is necessary to regulate fusion events during infection.