A Catalytic Role for Mod5 in the Formation of the Tea1 Cell Polarity Landmark

Abstract

SummaryMany systems regulating cell polarity involve stable landmarks defined by internal cues [1–5]. In the rod-shaped fission yeast Schizosaccharomyces pombe, microtubules regulate polarized vegetative growth via a landmark involving the protein Tea1 [6–9]. Tea1 is delivered to cell tips as packets of molecules associated with growing microtubule ends [10] and anchored at the plasma membrane via a mechanism involving interaction with the membrane protein Mod5 [11, 12]. Tea1 and Mod5 are highly concentrated in clusters at cell tips in a mutually dependent manner, but how the Tea1-Mod5 interaction contributes mechanistically to generating a stable landmark is not understood. Here, we use live-cell imaging, FRAP, and computational modeling to dissect dynamics of the Tea1-Mod5 interaction. Surprisingly, we find that Tea1 and Mod5 exhibit distinctly different turnover rates at cell tips. Our data and modeling suggest that rather than acting simply as a Tea1 receptor or as a molecular “glue” to retain Tea1, Mod5 functions catalytically to stimulate incorporation of Tea1 into a stable tip-associated cluster network. The model also suggests an emergent self-focusing property of the Tea1-Mod5 cluster network, which can increase the fidelity of polarized growth.

Chimera proteins with affinity for membranes and microtubule tips polarize in the membrane of fission yeast cells

“…After 30 min of MBC treatment, the average Tea1 signal at the cell ends was reduced and broadened, but very stable "hot spots" of Tea1 signal remained both at the cell ends and elsewhere along the cortex (Fig. S4) (5). To confirm the difference in response to MBC treatment under the exact same conditions, we further performed experiments in cells coexpressing Tea1-tdTomato and GFP-tagged chimera in the background of a tea1 deletion.…”

“…T he establishment of polarity involves the accumulation of signaling proteins at specific locations at the cell periphery (1)(2)(3). In fission yeast, the establishment of polarity patterns that control cell growth and division requires, in addition to the presence of microtubules (4), an elaborate regulation network with a large number of molecular components (5,6) (Fig. 1, Left).…”

A common mechanism for protein cluster formation

“…The comparison of the population-averaged concentration profile of Tea1 within the cell tips with a map of individual microtubule delivery events also registered in a population of cells (see Fig. 3E in the original work 10 ) gave us a hint that some mechanism, which effectively prevents aberrant deposition events from "smearing" the Tea1 profile, might be in operation. Indeed, while the majority of deposition events fall within close proximity of the tip center, every now and then a microtubule bundle leaves its cargo far from the center or even entirely outside the hemispheric dome of the cell tip.…”

“…As formulated, this mechanism bares similarities to will spread within the area with diameter 4 μm, the typical cross-section of a fission yeast cell. Given that the bundleassociated Tea1 packets arrive approximately once a minute, 10 the polarized delivery alone is not sufficient to account for the stable Tea1 cluster. To explain the existence of spatially-localized structures despite diffusive spreading, their components are often assumed to attach to either a pre-existing polymeric lattice, e.g., the acto-myosin cortex in mammals, or to be included in the de-novo formed polymer, e.g., clusters of death receptors in cells undergoing apoptosis 11 or glutamate receptors in the inhibitory synapses.…”

“…10 Mutual interdependence of Mod5 and Tea1 localization suggested a simple model in which Tea1 and Mod5 meet at the plasma membrane to form a stoichiometric complex that serves as a protomer for a polymeric coat-like structure. The resulting formation of a stable polymer follows the "diffusion-and-capture" mechanism introduced originally by Losick and colleagues 13 to describe formation of the spore coat in Bacillus subtilis.…”