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Characterization of Muscle Ankyrin Repeat Proteins in human skeletal muscle

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AJP Cell Physiology

Published online on

Abstract

Muscle Ankyrin Repeat Proteins (MARPs) are a family of titin-associated stress response molecules and putative transducers of stretch-induced signalling in skeletal muscle. In cardiac muscle, Cardiac Ankyrin Repeat Protein (CARP) and Diabetes-Related Ankyrin Repeat Protein (DARP) reportedly redistribute from binding sites on titin to the nucleus following prolonged stretch. However, it is unclear whether Ankyrin repeat-domain protein2 (Ankrd2) shows comparable stretch-induced redistribution to the nucleus. We measured in rested human skeletal muscle 1) the absolute amount of MARPs, and 2) the distribution of Ankrd2 and DARP in both single fibers and whole muscle preparations. In absolute amounts, Ankrd2 is the most abundant MARP in human skeletal muscle, there being ~3.1 µmol.kg-1, much greater than DARP and CARP (~0.11 and ~0.02 µmol.kg-1, respectively). All DARP was found to be tightly bound at cytoskeletal (or possibly nuclear) sites. In contrast, ~70% of the total Ankrd2 is freely diffusible in the cytosol (including virtually all the phosphorylated p-Ankrd2-Ser99 form), ~15% is bound to non-nuclear membranes and ~15% bound at cytoskeletal sites, likely at the N2A-region of titin. These data are not consistent with the proposal that Ankrd2 per se, or p-Ankrd2-Ser99, mediate stretch-induced signalling in skeletal muscle, dissociating from titin and translocating to the nucleus, because majority of these forms of Ankrd2 are already free in the cytosol. It will be necessary to show that the titin-associated Ankrd2 is modified by stretch in some as yet unidentified way, distinct from the diffusible pool, if it is to act as a stretch-sensitive signalling molecule.