The evolution of the actin binding NET superfamily.

Timothy J Hawkins

doi:10.3389/fpls.2014.00254

Abstract

The Arabidopsis Networked (NET) superfamily are plant-specific actin binding proteins which specifically label different membrane compartments and identify specialized sites of interaction between actin and membranes unique to plants. There are 13 members of the superfamily in Arabidopsis, which group into four distinct clades or families. NET homologs are absent from the genomes of metazoa and fungi; furthermore, in plantae, NET sequences are also absent from the genome of mosses and more ancient extant plant clades. A single family of the NET proteins is found encoded in the club moss genome, an extant species of the earliest vascular plants. Gymnosperms have examples from families 4 and 3, with a hybrid form of NET1 and 2 which shows characteristics of both NET1 and NET2. In addition to NET3 and 4 families, the NET1 and pollen-expressed NET2 families are found only as independent sequences in Angiosperms. This is consistent with the divergence of reproductive actin. The four families are conserved across Monocots and Eudicots, with the numbers of members of each clade expanding at this point, due, in part, to regions of genome duplication. Since the emergence of the NET superfamily at the dawn of vascular plants, they have continued to develop and diversify in a manner which has mirrored the divergence and increasing complexity of land-plant species.

Highlights

The Networked (NET) proteins are a superfamily of plant-specific actin-binding proteins which localize simultaneously to the actin cytoskeleton and specific membrane compartments and are suggested to couple these membranes to the actin cytoskeleton in plant cells (Deeks et al, 2012)
All of these protein families are absent from plants, despite actin-membrane interactions remaining critical for the plant cell with actin filaments dominating microtubules during organelle and endomembrane trafficking (Boevink et al, 1998; Kandasamy and Meagher, 1999; Van Gestel et al, 2002; Langowski et al, 2010)
There are several residues which are identical in all NET NET actin-binding (NAB) domains (W15W16W17,H20,S25,W27,L32,D51,A57,P65,R79, L81,A82) suggesting that, in addition to conserved motifs, these residues are likely to be essential for the structure of the domain and potentially its actin affinity

Summary

Introduction

The Networked (NET) proteins are a superfamily of plant-specific actin-binding proteins which localize simultaneously to the actin cytoskeleton and specific membrane compartments and are suggested to couple these membranes to the actin cytoskeleton in plant cells (Deeks et al, 2012). Evidence is accumulating that the plant cell employs analogous factors of its own, including those of the NET superfamily, to fulfill this role. In light of this plant specialization, it is rewarding to consider the evolutionary significance of the NET proteins and chart the development of the superfamily through plant evolution. Residues 1–94 of this 288 aa region are sufficient to associate directly with actin filaments This minimal actin binding region, referred to as the NET actin-binding (NAB) domain, represents a new actin binding motif unique to plants with no apparent primary sequence homology to previously identified actin binding domains. The C terminal domains of all NET proteins are predicted to take on a coiled-coil secondary structure which may provide an interface for proteinprotein interactions with itself, other NETs or additional binding partners (Deeks et al, 2012)

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