Isoenzyme-specific interaction of muscle-type creatine kinase with the sarcomeric M-line is mediated by NH2-terminal lysine charge-clamps

Creatine kinase (CK) is located in an isoenzyme-specific manner at subcellular sites of energy production and consumption. In muscle cells, the muscle-type CK isoform (MM-CK) specifically interacts with the sarcomeric hi-line, while the highly homologous brain-type CK isoform (BB-CK) does not share this property, Sequence comparison revealed two pairs of lysine residues that are highly conserved in M-CK but are not present in B-CIC. The role of these lysines in mediating hi-line interaction was tested with a set of M-CK and B-CK point mutants and chimeras. We found that all four lysine residues are involved in the isoenzyme-specific hi-line interaction? acting pair-wise as strong (K104/K115) and weak interaction sites (K8/K24). An exchange of these lysines in MM-CK led to a loss of M-line binding, whereas the introduction of the very same lysines into BB-CK led to a gain of function by transforming BB-CK into a fully competent M-line-binding protein. The role of the four lysines in MM-CK is discussed within the context of the recently solved x-ray structures of MM-CK and BB-CK.