Post-translational modifications, along with isoform splicing, of titin determine the passive tension development of stretched sarcomeres. It was recently shown that PKC alpha phosphorylates two highly-conserved residues (S26 and S170) of the PEVK region in cardiac titin, resulting in passive tension increase. To determine how each phosphorylated residue affects myocardial stiffness, we generated three recombinant mutant PEVK fragments (S26A, S170A and S170A/S26A), each flanked by Ig domains. Single-molecule force spectroscopy shows that PKC alpha decreases the PEVK persistence length (from 0.99 to 0.68 nm): the majority of this decrease is attributable to phosphorylation of S26. Before PKC alpha, all three mutant PEVK fragments showed at least 40% decrease in persistence length compared to wildtype. Furthermore. Ig domain unfolding force measurements indicate that PEVK's flanking Ig domains are relatively unstable compared to other titin Ig domains. We conclude that phosphorylation of S26 is the primary mechanism through which PKCa modulates cardiac stiffness. (C) 2010 Elsevier Inc. All rights reserved.