The Giardia Median Body Protein Is a Ventral Disc Protein That Is Critical for Maintaining a Domed Disc Conformation during Attachment

Abstract

Giardia has unique microtubule structures, including the ventral disc, the primary organelle of attachment to the host, and the median body, a structure of undefined function. During attachment, the ventral disc has a domed conformation and enables Giardia to attach to the host intestinal epithelia within seconds. The mechanism of attachment via the ventral disc and the overall structure, function, and assembly of the ventral disc are not well understood. Our recent proteomic analysis of the ventral disc indicated that the median body protein (MBP), previously reported to localize exclusively to the median body, was primarily localized to the ventral disc. Using high-resolution light and electron microscopy, we confirm that the median body protein localizes primarily to the overlap zone of the ventral disc. The MBP also occasionally localized to the median body during prophase. To define the contribution of MBP to the ventral disc structure, we depleted MBP using an anti-MBP morpholino. We found that the ventral disc was no longer able to form properly and that the disc structure often had an aberrant nondomed or flattened horseshoe conformation. The ability of attached anti-MBP morpholino-treated trophozoites to withstand shear forces and normal forces was significantly decreased. Most notably, the plasma membrane contacts with the surface, including those of the bare area, were defective after the anti-MBP knockdown. To our knowledge, this is the first ventral disc protein whose depletion directly alters ventral disc structure, confirming that the domed ventral disc conformation is important for robust attachment.