To analyze potential functional consequences of myosin heavy chain (MHC) 
mutations identified in patients with familial hypertrophic cardiomyopathy 
(FHC), we have assessed the stability of the mutant MHCs and their ability to 
form thick filaments. Constructs encoding wild-type rat alpha MHC and seven 
corresponding FHC missense mutants were transfected into COS cells. Immunoblot 
analysis suggested that FHC mutations do not grossly alter protein stability. 
Wild-type alpha MHC transfected into COS cells forms structures previously shown 
to be arrays of thick filaments, which also resemble myosin structures observed 
early in differentiation of muscle cells. Surprisingly, up to 29% of COS cells 
transfected with the FHC mutants failed to form filamentous structures. To 
assess whether this phenotype was specific for the FHC mutants and not 
generalizable to any myosin mutation, COS cells were transfected with a 
construct encoding an MHC with a 168-amino acid deletion of the hinge/rod 
region. This deletion construct formed filamentous structures with the same 
frequency as wild-type MHC. Biochemical analysis of one FHC mutant 
(Arg-249-->Gln) demonstrates that the structures formed by the mutant are 
solubilized at a lower ionic strength than those formed by wild-type MHC. We 
conclude that although the FHC mutant MHC is not labile, its assembly properties 
may be impaired.