The architecture, assembly, and complexes of sarcomeric proteins

(last update January 2007)


Introduction

Titin, the largest gene product of the human genome, comprises up to 38,000 residues in its largest isoform and is organised into about 300 protein domains, of which the majority are folded as immunoglobulin (Ig)-like domains or fibronectin type-III (FN-III)-like domains (Figure 1). It is known as the third filament of the muscle sarcomere and is involved in multiple functions, such as acting as a "molecular ruler" to align major components of the sarcomere, as well as muscle development, passive elasticity, and muscle signalling.
 

 


Figure 1: Domain structure of titin  (M.L. Bang et al. (2001) Circ. Res. 89, 1065).


Our Achievements

Our group was first mainly interested in providing high-resolution structures of some representative titin domains. More recently, our focus has shifted to investigate the structural/functional basis of the involvement of selected titin domain or multi-domain fragments in interactions with other sarcomeric components. Building on our early data of the structure of the serine/threonine kinase domain of titin (Mayans et al. 1998), we mostly focused on investigating the architecture of titin and its interactions with protein components within the central A/M-band region and the distal Z-disk of muscle sarcomeres.

More than a decade's efforts have led to the identification of a purification protocol that has allowed us to crystallise the N-terminal assembly complex of titin in the absence and presence of the Z-disk protein telethonin (Zou et al. 2003; Marino et al. 2006; Zou et al. 2006). To our surprise, the structure revealed a (2:1) sandwich complex, characterised by two titin molecules assembled by telethonin, acting as a mediator, with pseudo-palindromic symmetry (Figure 2). Folding of the N-terminal part of telethonin is induced by the presence of titin, which has made structural analysis of separate telethonin impossible, to date. In order to independently validate our structural findings, we developed a protocol to site-specifically label the titin/telethonin complex to allow a distance-sensitive FRET analysis.


 


Figure 2: Surface presentation of the titin/telethonin (2:1) complex


The following further structures were determined by our group during the reporting period: the Ig-tandem arrangement A168-A169 of titin, near the titin kinase domain (Mueller et al., submitted); the M1 Ig-like domain of titin, C-terminal to the titin kinase domain (Mueller et al., unpublished); the PB1 domain of the titin kinase downstream signalling molecule NBR1 (Lange et al. 2005; Muller et al. 2006); the PB1/PB1 domain complex of the downstream signalling components NBR1 and P62 (Mueller et al., unpublished), and the I27 Ig-like domain from the I-band of titin (Vega et al., unpublished).


Future Perspectives

We are planning to focus on the assembly of sarcomeric protein and their interactions with various ligands (Figure 3). The identification of the molecular basis of these interactions will allow us to map and to interprete available disease mutations. We will specifically investigate titin (continued),the M-band protein myomesin and the Z-disk anchored filaments nebulin and nebulette.



Figure 3: Model outlining the architecture of the sarcomeric Z-disk. Titin filaments are assembled by a dual Z-disk bridging system, by a-actinin rods on a variable number of titin Z-repeats [three bridges are shown], and by telethonin via the N-terminal IG domains Z1 and Z2.


Our Collaborators


External Support


Relevant Publications from our Laboratory


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