Involved in the organization of myofibers. Together with KRT8, helps to link the contractile apparatus to dystrophin at the costameres of striated muscle (By similarity). Heterotetramer of two type I and two type II keratins. Interacts with PNN and the actin-binding domain of DMD (By similarity). This keratin differs from all other IF proteins in lacking the C-terminal tail domain. There are two types of cytoskeletal and microfibrillar keratin: I (acidic; 40-55 kDa) and II (neutral to basic; 56-70 kDa). Belongs to the intermediate filament family.