BIOLOGICAL SIGNIFICANCE OF PHOSPHORYLATION AND MYRISTOYLATION IN THE REGULATION OF CARDIAC-MUSCLE PROTEINS

Post-translational modification has long been recognized as a way in w hich the properties of proteins may be subtly altered after synthesis of the polypeptide chain is complete. Amongst the moieties most common ly encountered covalently attached to proteins are oligosaccharides, p hosphate, acetyl, formyl and nucleosides. Protein phosphorylation and dephosphorylation is one of the most prevalent and best understood mod ifications employed in cellular regulation. The bovine heart calmoduli n-dependent cyclic nucleotide phosphodiesterase (CaMPEDE) can be phosp horylated by cAMP-dependent protein kinase, resulting in a decrease in the enzyme's affinity for Ca2+ and calmodulin (CaM). The phosphorylat ion of CaMPDE is blocked by Ca2+ and CaM and reversed by the CaM-depen dent phosphatase (calcineurin). The dephosphorylation is accompanied b y an increase in the affinity of the phosphodiesterase for CaM. Analys is of the complex regulatory properties of CaMPDE has led to the sugge stion that fluxes of cAMP and Ca2+ during cell activations are closely coupled and that the CaMPDE play a key role in the signal coupling ph enomenon. The high molecular weight calmodulin binding protein (HMWCaM BP) was phosphorylated by cAMP-dependent protein kinase. Phosphorylati on of HMWCBP was higher in the absence of Ca2+/CaM then in the presenc e of Ca2+/CaM and reversed by the CaM-dependent phosphatase. Recently, it has become apparent that the binding of myristate to proteins is a lso widespread in eukaryotic cells and viruses and certainly is of gre at importance to the correct functioning of an organism. Myristoyl CoA :protein N-myristoyltransferase (NMT) catalyses the attachment of myri state to the amino-terminal glycine residue of various signal transduc tion proteins. Cardiac tissue express high levels of cAMP-dependent pr otein kinase whose catalytic subunit is myristoylated. The subcellular localization of bovine cardiac muscle NMT indicated a majority of the activity was localized in cytoplasm. Under native conditions the enzy me exhibited an apparent molecular mass of 50 kDa. Recovery of NMT act ivity, from both cytosol and particulate fractions, was found to be hi gher than the total activity in crude homogenates, suggesting that par ticulate fraction may contain an inhibitory activity towards NMT. Rese arch in our laboratory has been focusing on the covalent modification of proteins and regulation of various signal transduction proteins. Th is special review is designed to summarize some aspects of the current work on co- and post-translational modification of proteins in cardia c muscle.