The mechano-sensing role of the unique SH3 insertion in plakin domains revealed by Molecular Dynamics simulations

Csaba Daday,Katra Kolšek,Frauke Gräter

doi:10.1038/s41598-017-11017-2

Csaba Daday, Katra Kolšek + Show 1 more

Open Access

https://doi.org/10.1038/s41598-017-11017-2

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Abstract

The plakin family of proteins, important actors in cross-linking force-bearing structures in the cell, contain a curious SH3 domain insertion in their chain of spectrin repeats (SRs). While SH3 domains are known to mediate protein-protein interactions, here, its canonical binding site is autoinhibited by the preceding SR. Under force, however, this SH3 domain could be released, and possibly launch a signaling cascade. We performed large-scale force-probe molecular dynamics simulations, across two orders of magnitude of loading rates, to test this hypothesis, on two prominent members of the plakin family: desmoplakin and plectin, obligate proteins at desmosomes and hemidesmosomes, respectively. Our simulations show that force unravels the SRs and abolishes the autoinhibition of the SH3 domain, an event well separated from the unfolding of this domain. The SH3 domain is free and fully functional for a significant portion of the unfolding trajectories. The rupture forces required for the two proteins significantly decrease when the SH3 domain is removed, which implies that the SH3 domain also stabilizes this junction. Our results persist across all simulations, and support a force-sensing as well as a stabilizing role of the unique SH3 insertion, putting forward this protein family as a new class of mechano-sensors.

Highlights

Mechanotransduction is known to be an important biological process rendering tissues both resilient and responsive towards their mechanical environment
FPMD26, known as steered MD27, has been used to study the individual unfolding of both SRs11, 13, 14 and SH3s17, often in conjunction with experiments. We will apply this method to different constructs of desmoplakin and plectin (Fig. 1) to answer the following questions: (i) do the domains unfold simultaneously or with well-delimited force peaks, (ii) would the absence of the SH3 domain significantly change the nature of the unfolding pathways or the rupture forces, (iii) is the SH3 domain activated under force or does it unfold while autoinhibited, and (iv) can we identify relevant differences between the two considered proteins? Our simulation data suggests that the SH3 domain both stabilizes the spectrin repeats against unfolding and is force-activated for downstream signaling
While the two plakin family members studied by force-probe molecular dynamics (FPMD) simulations share the major unfolding mechanism, we find desmoplakin to unfold at higher forces and in more distinct steps than plectin, shedding light on their mechanotransduction role in their different cell junction environments

Summary

Introduction

Mechanotransduction is known to be an important biological process rendering tissues both resilient and responsive towards their mechanical environment. The plakin family of cytolinker proteins[4,5,6] has long-established physiological relevance in several different biological tissues Proteins in this family share a central plakin domain, consisting mainly of spectrin repeat domains. Spectrin repeats[7] (SRs) are the main building blocks of structural proteins, consisting of three helices A, B, and C (A and C being parallel and antiparallel to B) and generally containing about 100–120 residues in total. Their overall sequence identity is quite low, even when considering repeats from the same protein[8, 9].

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