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A tale of two polymers: new insights into helical filaments

Nature Reviews Molecular Cell Biology volume 4, pages 621–631 (2003)Cite this article

Key Points

  • General insights into helical protein polymers can be found from studies of actin (in eukaryotes) and RecA (in bacteria), each of which can be the most abundant protein in a cell.

  • Actin and RecA are each part of two different superfamilies of structurally conserved proteins. Although actin and RecA both form helical polymers, most of the other superfamily members do not, indicating that small differences have allowed these proteins to function as polymers.

  • Many proteins, including actin and RecA, can readily assemble in vitro into different higher-order assemblies, which makes it difficult to determine the assembly state of the protein in the cell. Alternatively, the multiple interfaces that can be found in vitro might reflect different interactions that have a physiological relevance.

  • Actin subunits are seen with multiple states of 'twist' within a filament, and a new form of actin, involving tilted subunits, shows that different subunit–subunit interfaces must be able to co-exist within the same filament.

  • Multiple states of the RecA polymer, including an activated and an inactivated form, also seem to arise from multiple subunit–subunit interfaces.

  • Both allosteric and cooperative effects exist within actin and RecA filaments, indicating that these protein polymers have evolved to function as more than merely an ensemble of independent subunits.

Abstract

Many proteins function as helical polymers within the cell. Two intensively studied examples are eukaryotic actin and bacterial RecA, which belong to two different protein superfamilies. However, most other members of these superfamilies do not polymerize into helical filaments. General features of polymorphism, cooperativity and allostery that emerge from studies of eukaryotic actin and bacterial RecA raise more general issues about how conserved these filamentous structures have been during evolution.

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Figure 1: Properties of the actin subunit.
Figure 2: Comparison of F-actin and T-actin.
Figure 3: Different states of RecA.

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  1. Department of Biochemistry and Molecular Genetics, University of Virginia, Charlottesville, 22908-0733, Virginia, USA

    Edward H. Egelman

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DATABASES

Biomachina

Holmes model

Entrez

RadA

Interpro

HSP-70

LocusLink

Actin

RAD51

Protein Data Bank

Arp3

FtsA

FtsZ

hexokinase

ParM

RecA

T7 gp4

Tubulin

Swiss-Prot

DMC1

MreB

RAD51D

XRCC2

Glossary

TUBULIN

The protein that polymerizes to form microtubules.

NUCLEATION

Polymer growth can be separated into nucleation (the formation of a 'seed' that will allow a polymer to grow) and elongation (when subunits add on to an existing nucleus or filament).

COFILIN

A member of the actin-depolymerizing factor (ADF)/cofilin family of proteins, which are involved in the depolymerization of actin filaments in the cell.

HELICASE

A protein that uses the energy provided by ATP hydrolysis to open double-stranded DNA, double-stranded RNA or RNA–DNA hybrids, into two separate strands.

MYOFIBRILS

Muscle fibres are composed of myofibrils, primarily containing actin and myosin.

CYTOPLASMIC STRESS FIBRES

Contractile bundles of actin and myosin that are attached to sites of adhesion between a cell and the substrate.

PHALLOIDIN

A toxin that binds to, and stabilizes, F-actin.

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Egelman, E. A tale of two polymers: new insights into helical filaments. Nat Rev Mol Cell Biol 4, 621–631 (2003). https://doi.org/10.1038/nrm1176

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