Functional and structural similarities between the inhibitory region of troponin I coded by exon VII and the calmodulin-binding regulatory region of the catalytic subunit of phosphorylase kinase.

Abstract

A sequence homology has been noted between the carboxyl quarter of the catalytic y subunit ofphosphorylase kinase and the region of troponin I coded by exon VII. Because this portion of troponin I contains the inhibitory region that interacts with actin and troponin C, we have emined whether the y subunit of phosphorylase kinase can functionally mimic troponin I by also interacting with actin and troponin C. We have found that troponin C not only activates the isolated y subunit of phosphorylase kinase but also binds with approximately the same affinity as calmodulin. Although actin had no effect on the activity of the y subunit alone, it did inhibit the activity of y-calmodulin and y-troponin C complexes. Conversely, the y subunit was able to inhibit actomyosin ATPase in a process that could be overcome by calmodulin. These results suggest that actin and calmodulin (or troponin C) compete for binding to the y subunit. Moreover, the structural and functional similarities between the y subunit and troponin I suggest that the y subunit of phosphorylase kinase may have evolved from the fusion of a protein kinase protogene with a progenitor of exon VII of troponin I.

Evidence for the Location of the Allosteric Activation Switch in the Multisubunit Phosphorylase Kinase Complex from Mass Spectrometric Identification of Chemically Crosslinked Peptides

“…TnC and CaM are Ca 2+ -binding homologs, 48 and TnI and γ share structural and functional similarities. 5,47 Regarding the latter pair, we have demonstrated that the isolated γ subunit of PhK binds CaM and TnC with equal affinity, and that a discrete region of the γCRD (residues 294-309) bears remarkable sequence similarity to the inhibitory region of TnI encoded by exon VII, 47 a region that has been shown to bind TnC. 49 On the basis of our current observation that K303 in the TnI-like region of γ ( 302 GKFK) is crosslinked to the inhibitory 1-22 terminus of the β subunit in the PhK complex, and that in the Tn complex TnI is known to bind TnT in addition to TnC, we screened the sequence of β against those of TnT and TnI for the best "nonintersecting" alignments that might suggest additional, structurally similar regions in the two complexes.…”

“…19,21,22,36 It has been noted that the skeletal muscle PhK complex and the actin·troponin (Tn) complex system have evolved similar mechanisms through which their functions are regulated by Ca 2+ . 47 The heterotrimeric Tn complex is composed of troponin T (TnT), troponin I (TnI) and troponin C (TnC). TnC and CaM are Ca 2+ -binding homologs, 48 and TnI and γ share structural and functional similarities.…”

Electrostatic changes in phosphorylase kinase induced by its obligatory allosteric activator Ca2+

“…Perhaps even more relevant to the PhK complex is the switch involving a group of critical residues that participate in the charge-charge interactions between the inhibitory region of cardiac troponin I (cTnI) and its cognate linker helix region of cardiac troponin C (cTnC) (Lindhout et al 2005). The cTnI subunit shares a high degree of sequence similarity with the C-terminal regulatory domain of PhKg (Paudel and Carlson 1990), which has been shown to interact with CaM (Dasgupta et al 1989;Harris et al 1990), PhK's intrinsic d subunit. This subunit is, in turn, a homolog of cTnC, the Ca 2+ -binding subunit of the troponin complex.…”

Expressed Phosphorylase b Kinase and Its αγδ Subcomplex as Regulatory Models for the Rabbit Skeletal Muscle Holoenzyme

“…Screening of the γ CRD for potential high-affinity CaM-binding sequences through the use of overlapping synthetic peptides revealed two distinct, nonoverlapping CaM-binding domains (55). The more N-terminal of these contains regions of high sequence similarity to skeletal muscle troponin I (55–57), including its functional “inhibitory peptide” (58) that interacts with troponin C or actin in a Ca 2+ -dependent manner (59, 60). Through zero-length cross-linking, we have shown an Asp-Lys salt bridge within the PhK complex between δ and the TnI-like domain of γ (32).…”