A novel interaction of cGMP-dependent protein kinase I with troponin T

cGMP-dependent protein kinase (cGK) is a major intracellular receptor of cG MP and is implicated in several signal transduction pathways. To identify p roteins that participate in the cGMP/cGK signaling pathway, we employed the yeast two-hybrid system with cGK I alpha as bait. cDNAs encoding slow skel etal troponin T (skTnT) were isolated from both mouse embryo and human skel etal muscle cDNA libraries, The skTnT protein interacted with cGK I beta bu t not with cGK II nor cAMP-dependent protein kinase, The yeast two-hybrid a nd in vitro binding assays revealed that the N-terminal region of cGK I alp ha, containing the leucine zipper motif, is sufficient for the association with skTnT, In vivo analysis, mutations in cGK I alpha, which disrupted the leucine zipper motif, were shown to completely abolish the binding to skTn T. Furthermore, cGK I also interacted with cardiac TnT (CTnT) but not with cardiac troponin I (cTnI), Together with the observations that cTnI is a go od substrate for cGK I and is effectively phosphorylated in the presence of cTnT in vitro, these findings suggest that TnT functions as an anchoring p rotein for cGK I and that cGK I may participate in the regulation of muscle contraction through phosphorylation of TnI.