Muscle assembly: a titanic achievement?

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Abstract

The formation of perfectly aligned myofibrils in striated muscle represents a dramatic example of supramolecular assembly in eukaryotic cells. Recently, considerable progress has been made in deciphering the roles that titin, the third most abundant protein in muscle, has in this process. An increasing number of sarcomeric proteins (ligands) are being identified that bind to specific titin domains. Titin may serve as a molecular blueprint for sarcomere assembly and turnover by specifying the precise position of its ligands within each half-sarcomere in addition to functioning as a molecular spring that maintains the structural integrity of the contracting myofibrils.

Introduction

The sarcomere is the contractile unit of striated muscle. Its formation is a unique example in biology where thousands of molecules assemble into a supramolecular structure with almost crystalline order. As a result of this precise assembly, many different classes of proteins function together to efficiently convert the molecular-level movements produced by actin and myosin into macroscopic movements of muscle.

It has been speculated that, in vertebrates, the giant filamentous protein titin (also known as connectin), the only known structure that spans the entire half-sarcomere, may act as a molecular blueprint to direct sarcomere assembly (e.g.[1]; see Figure 1). In this role, titin would coordinate the precise assembly of the structural, regulatory and contractile proteins within the sarcomere, a prerequisite for efficient muscle contraction. In the invertebrate Caenorhabditis elegans, different assembly mechanisms may have developed, since the thick filaments in their obliquely striated muscles are much longer than those in vertebrates, and no molecule of titin-like size has yet been identified.

Several previous reviews have focused on the general properties, isoforms and order of assembly of the known sarcomeric components 2, 3, 4. Here we emphasize the more recent developments in our knowledge of the possible roles that titin has in myofibril assembly. We first discuss the molecular structure of the titin filament and its detailed layout within the sarcomere. Next, we review progress in identifying specific protein ligands of titin. Finally, we speculate on possible mechanisms involved in orchestrating myofibril assembly and maintenance.

Section snippets

Molecular structure of the titin filament

A decade ago, there was much resistance to the idea that a eukaryotic cell could translate megadalton-sized ‘titanic’ polypeptides; however, it is now firmly established that this is possible as a full-length titin cDNA was isolated that encodes a polypeptide of 27,000 residues [5]. A single gene locus for titin was located on the long arm of chromosome 2 in humans (e.g. [6]). This gene is transcribed to produce a full-length mRNA of up to 100 kilobases, which encodes a titin polypeptide of up

Layout of the titin filament in sarcomeres

Initial immuno-electron microscopy studies, with a set of 10 nonrepetitive anti-titin monoclonal antibodies, revealed that a single molecule of titin extends from the Z-line to close to the M-line region [9]. More detailed information on the layout of the titin filament within the sarcomere has been obtained recently by immuno-electron microscopy studies using antibodies raised against specific bacterially-expressed domains of titin (see Figure 1, Figure 2).

Interaction of titin with other sarcomeric components

Here, we describe the proteins that interact with titin —starting from its amino-terminal Z-disc to its carboxy-terminal M-line end — and identify their potential binding sites in human titin sequences (see Figure 2).

Differential expression of titin

Only a single copy of the titin gene has been identified in vertebrate genomes, but many distinct isoforms are generated by differential splicing (Figure 2).

Titin as a molecular template in myofibril assembly

One of the challenges of muscle research is to identify the sequence of events involved in assembling thick and thin filaments and Z-lines to form sarcomeres. The direct interaction of titin with various sarcomeric proteins and the presence of many titin isoforms are consistent with its proposed role as a molecular template during myofibril assembly. Interestingly, studies have suggested that thick and thin filaments, assemble independently (for recent discussion, see [44]). The coordinated

Acknowledgements

We especially thank Thomas Centner for the illustrations. We also thank the many members of our (past and present) laboratories especially Bernhard Kolmerer, Karoly Trombitás, and Thomas Centner who have made critical contributions, both scientifically and intellectually, to the development of this review. Work in our laboratories is supported by grants from the Human Frontier Science Program (Carol Gregorio, Hiroyuki Sorimachi, Siegfried Labeit), National Institutes of Health HL57461 (Carol

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

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