Previous work has shown that mutations in muscle LIM protein (MLP) can cause 
hypertrophic cardiomyopathy (HCM). In order to gain an insight into the 
molecular basis of the disease phenotype, we analysed the binding 
characteristics of wild-type MLP and of the (C58G) mutant MLP that causes 
hypertrophic cardiomyopathy. We show that MLP can form a ternary complex with 
two of its previously documented myofibrillar ligand proteins, N-RAP and 
alpha-actinin, which indicates the presence of distinct, non-overlapping binding 
sites. Our data also show that, in comparison to wild-type MLP, the capacity of 
the mutated MLP protein to bind both N-RAP and alpha-actinin is significantly 
decreased. In addition, this single point mutation prevents zinc coordination 
and proper folding of the second zinc-finger in the first LIM domain, which 
consequently renders the protein less stable and more susceptible to 
proteolysis. The molecular basis for HCM-causing mutations in the MLP gene might 
therefore be an alteration in the equilibrium of interactions of the ternary 
complex MLP-N-RAP-alpha-actinin. This assumption is supported by the previous 
observation that in the pathological situation accompanied by MLP down 
regulation, cardiomyocytes try to compensate for the decreased stability of MLP 
protein by increasing the expression of its ligand N-RAP, which might finally 
result in the development of myocyte disarray that is characteristic of this 
disease.