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We have analyzed the interactions between two types of sarcomeric proteins: myosin heavy chain (MyHC) and members of an abundant thick filament-associated protein family (myosin-binding protein; MyBP). Previous work has demonstrated that when MyHC is transiently transfected into mammalian nonmuscle COS cells, the expressed protein forms spindle-shaped structures consisting of bundles of myosin thick filaments. Co-expression of MyHC and MyBP-C or -H modulates the MyHC structures, resulting in dramatically longer cables consisting of myosin and MyBP encircling the nucleus. Immunoelectron microscopy indicates that these cable structures are more uniform in diameter than the spindle structures consisting solely of MyHC, and that the myosin filaments are compacted in the presence of MyBP. Deletion analysis of MyBP-H indicates that cable formation is dependent on the carboxy terminal 24 amino acids. Neither the MyHC spindles nor the MyHC/MyBP cables associate with the endogenous actin cytoskeleton of the COS cell. While there is no apparent co-localization between these structures and the microtubule network, colchicine treatment of the cells promotes the formation of longer assemblages, suggesting that cytoskeletal architecture may physically impede or regulate polymer formation/extension. The data presented here contribute to a greater understanding of the interactions between the MyBP family and MyHC, and provide additional evidence for functional homology between MyBP-C and MyBP-H.