Poly-Ig tandems from I-band titin share extended domain arrangements irrespective of the distinct features of their modular constituents

Abstract

The cellular function of the giant protein titin in striated muscle is a major focus of scientific attention. Particularly, its role in passive mechanics has been extensively investigated. In strong contrast, the structural details of this filament are very poorly understood. To date, only a handful of atomic models from single domain components have become available and data on poly-constructs are limited to scarce SAXS analyses. In this study, we examine the molecular parameters of poly-Ig tandems from I-band titin relevant to muscle elasticity. We revisit conservation patterns in domain and linker sequences of I-band modules and interpret these in the light of available atomic structures of Ig domains from muscle proteins. The emphasis is placed on features expected to affect inter-domain arrangements. We examine the overall conformation of a 6Ig fragment, I65-I70, from the skeletal I-band of soleus titin using SAXS and electron microscopy approaches. The possible effect of highly conserved glutamate groups at the linkers as well as the ionic strength of the medium on the overall molecular parameters of this sample is investigated. Our findings indicate that poly-Ig tandems from I-band titin tend to adopt extended arrangements with low or moderate intrinsic flexibility, independently of the specific features of linkers or component Ig domains across constitutively- and differentially-expressed tandems. Linkers do not appear to operate as free hinges so that lateral association of Ig domains must occur infrequently in samples in solution, even that inter-domain sequences of 4-5 residues length would well accommodate such geometry. It can be expected that this principle is generally applicable to all Ig-tandems from I-band titin.