Immunoglobulin-type domains of titin are stabilized by amino-terminal extension.

We have recently suggested that similarly folded titin modules located at different sarcomeric regions have distinct molecular properties and stability. Could our selection of module boundaries have potentially influenced our conclusions? To address this question we expressed amino-terminally extended versions of the same modules and determined, with the use of CD and Fluorescence techniques, key thermodynamic parameters characterizing their stability. We present here our results which confirm our previous observations and show that, while amino-terminal extension has a profound effect on the stability of individual modules, it does not affect at all their folding pattern or their relative stabilities. Moreover, our data suggest that the selection of module boundaries can be of critical importance for the structural analysis of modular proteins in general, especially when a well-defined intron-exon topography is absent and proteolytic methods are inconclusive.