Adhesive modular proteins occur in the extracellular mucilage of the motile, pennate diatom Phaeodactylum tricornutum

Abstract

This Letter reports on adhesive modular proteins recorded by atomic force microscopy on live cells from the extracellular mucilage secreted from, and deposited around, the motile form of the pennate diatom Phaeodactylum tricornutum. This is the first report of modular proteins and their supramolecular assemblies, called adhesive nanofibers (ANFs), to be found on diatoms that use adhesives not only for substratum adhesion, but as a conduit for cell motility. The permanent adhesive pads secreted by Toxarium undulatum, a sessile centric diatom, were previously shown to possess ANFs with a modular protein backbone. Our results reported here suggest that modular proteins may be an important component of diatom adhesives in general, and that diatoms utilize the tensile strength, toughness, and flexibility of ANFs for multiple functions. Significantly, the genome of P. tricornutum has recently been sequenced; this will allow directed searches of the genome to be made for genes with modular protein homologs, and subsequent detailed studies of their molecular structure and function.

FIGURE 1

Force versus separation curves recorded from the surface of stationary ovoid P. tricornutum cells representing ANFs (top) and a single modular protein (bottom).

FIGURE 2

Histograms showing the distribution of peak forces (left) and distance between peaks (right) for the sawtooth curves recorded from the extracellular matrix of P. tricornutum. Note there are peaks that have much shorter or greater spacing; these occur within traces where the remainder of the peaks have normal spacing, i.e., ∼120 nm. These can be explained by either modular proteins that are misfolded, or that don't unfold evenly, due to interactions with surrounding fibers and/or diatom extracellular mucilage.

FIGURE 3

Average peak force versus average persistence length measured from each of the sawtooth curves recorded from the extracellular matrix of P. tricornutum.