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Arp2/3 and SCAR: plants move to the fore

Nature Reviews Molecular Cell Biology volume 6, pages 954–964 (2005)Cite this article

Key Points

  • In animals and protists the Arp2/3 complex is required for the formation of membrane protrusions, such as lamellipodia and filopodia, which are essential for cell motility.

  • In yeast and animals, the Arp2/3 complex also drives actin assembly, which is associated with endocytotic vesicle formation. Yeast growth is severely disabled in the absence of Arp2/3-complex-powered endocytosis.

  • Unlike yeast and animals, plants continue to thrive with null-mutant alleles of Arp2/3-complex subunits. Epidermal cells of mutant plants show subtle morphogenetic phenotypes.

  • The SCAR/WAVE protein complex regulates Arp2/3-complex activation in the context of animal and protist cell motility. Other Arp2/3-complex activators, such as WASP, regulate endocytotic-vesicle formation.

  • Although plant cells form neither lamellipodia nor filopodia, plants retain genes encoding the components of the SCAR/WAVE complex. Plant homologues of most other known Arp2/3-complex regulators have not yet been identified.

  • Null mutant alleles of SCAR/WAVE complex proteins can phenocopy Arp2/3-complex mutants, implying that the SCAR/WAVE complex is the main regulator of the Arp2/3 complex in plants. In the absence of cell motility, plants must exploit the relationship between the SCAR/WAVE complex and the Arp2/3 complex in a novel cellular context that could be relevant for all eukaryotes.

Abstract

The actin-nucleating Arp2/3 complex is essential for life in yeast and animals, but not in plants, in which mutants of Arp2/3 complex components show relatively minor developmental abnormalities. Animal cells control the activity of the Arp2/3 complex through the suppressor of cyclic AMP receptor (SCAR) complex to achieve cell motility. Amazingly, plants have also retained the SCAR cell-motility pathway, and now provide a unique model for the study of new aspects of SCAR function in the absence of cell motility.

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Figure 1: Plant F-actin.
Figure 2: The Arp2/3 complex.
Figure 3: Branching F-actin in lamellipodia.
Figure 4: A. thaliana distorted trichomes.
Figure 5: SCAR-complex binding partners.

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Authors and Affiliations

  1. The Integrative Cell Biology Laboratory, School of Biological and Biomedical Sciences, University of Durham, South Road, Durham, DH1 3LE, UK

    Michael J. Deeks & Patrick J. Hussey

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  1. Michael J. Deeks
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  2. Patrick J. Hussey
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Correspondence to Patrick J. Hussey.

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DATABASES

Entrez Gene

Arp2

Arp3

BRICK1

WRP

TAIR

ARPC4

ARPC5

Glossary

PROTIST

A single-celled eukaryote.

BARBED END

An actin polymer has two ends; the barbed (+) end and the pointed (−) end. The barbed end has a higher affinity for monomers and grows at a faster rate.

FORMIN FAMILY

A family of multidomain scaffold proteins that are involved in actin-dependent morphogenetic events. They are conserved from plants to humans and are characterized by the presence of two conserved carboxy-terminal regions: the formin homology (FH) domains FH1 and FH2.

X-RAY FIBRE DIFFRACTION

The exposure of an ordered array of molecular fibres to an X-ray beam in order to obtain a diffraction pattern that, in turn, can be used to calculate a high-resolution molecular model of the fibre.

DISASSOCIATION EQUILIBRIUM CONSTANT (Kd)

Kd is calculated as the ratio of the dissociation and association rate constants of two molecules undergoing a reversible interaction. The unit of the disassociation constant is moles per litre. A low value for Kd represents a relatively strong association. Consequently, the calculated Kd between an actin monomer and another actin monomer is high, whilst the Kd between an actin monomer and an F-actin polymer is low. For F-actin of three or more subunits, Kd is equal to the critical concentration of actin polymerization.

RING CANALS

Cytoplasmic connections between D. melanogaster nurse cells and developing oocytes.

LAMELLIPODIA

Broad, flat protrusions at the leading edge of a moving cell that are enriched with a branched network of actin filaments.

LEADING EDGE

The thin margin of a lamellipodium that spans the area of the cell from the plasma membrane to a depth of about 1 μm into the lamellipodium.

FILOPODIA

Thin, transient actin protrusions that extend out from the cell surface and are formed by the elongation of bundled actin filaments in the core.

CLATHRIN-COATED VESICLE (CCV).

Transport vesicles that bud with the help of a coat protein known as clathrin.

FORWARD GENETICS

A genetic analysis that proceeds from phenotype to genotype by positional cloning or candidate-gene analysis.

REVERSE GENETICS

Genetic analysis that proceeds from genotype to phenotype through gene-manipulation techniques.

PAVEMENT CELLS

Plant leaf epidermal cells that coordinate through a series of lobed extensions to produce a continuous interlocking surface.

SINGLE PARTICLE AVERAGING

The generation of a model of a macromolecule through the analysis of multiple transmission electron microscope images (often thousands).

RHO-FAMILY GTPASE

A Ras-related GTPase that is involved in controlling the polymerization of actin.

EFFECTOR

A protein or protein complex that binds the GTPase directly and in a GTP-dependent manner and is required for the downstream function determined by that GTPase.

GUANINE NUCLEOTIDE-EXCHANGE FACTOR (GEF)

A protein that facilitates the exchange of GDP (guanine diphosphate) for GTP (guanine triphosphate) in the nucleotide-binding pocket of a GTP-binding protein.

MONOCOTYLEDON

A plant that has a single cotyledon (seed leaf) as an embryo.

DICOTYLEDON

One of the two principal classes of flowering plant, dicots are characterized by two cotyledons (primitive leaves) in the embryonic plant. Tomatoes, maple trees and mustard are common dicots.

TIP-GROWING CELLS

Cells of plants and fungi that grow by depositing new cell wall and membrane material at a focused polar site (the tip).

BREFELDIN A

A drug that is thought to disrupt endocytotic processes by inhibiting GTPase exchange factors for ADP-ribosylation factor (ARF) GTPases. ARFs function in many processes that include Golgi COPI vesicle formation.

FLUID-PHASE ENDOCYTOSIS

Fluid-phase endocytosis is the nonspecific uptake of fluid at the cell membrane. This contrasts receptor-mediated endocytosis where a ligand-receptor interaction triggers active uptake.

LUCIFER YELLOW

A soluble fluorescent chemical used to observe fluid-phase endocytosis.

EXPRESSED SEQUENCE TAGS (ESTs).

Short DNA sequences (several hundred base pairs) that are produced by reverse transcription of mRNA into DNA. ESTs are cDNAs that consist of exons and the sequences that flank exons. The sequencing of ESTs allows rapid identification ('tagging') of genes and can expedite DNA-marker development in coding genes.

CONTIG

Refers to an overlapping set of DNA fragments, in this context the overlapping series of genomic clones that reconstitute the five A. thaliana chromosomes.

EPSIN HOMOLOGY DOMAIN

A family of protein–protein interaction domains that are frequently found in endocytic proteins.

CORTACTIN

An animal Arp2/3-complex-activating protein that is recruited to lamellipodia.

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Deeks, M., Hussey, P. Arp2/3 and SCAR: plants move to the fore. Nat Rev Mol Cell Biol 6, 954–964 (2005). https://doi.org/10.1038/nrm1765

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