Abstract
Muscle contractile proteins are expressed as a series of developmental isoforms that are in constant dynamic remodeling during embryogenesis, but how obsolete molecules are recognized and removed is not known. Ozz is a developmentally regulated protein that functions as the adaptor component of a RING-type ubiquitin ligase complex specific to striated muscle. Ozz−/− mutants exhibit defects in myofibrillogenesis and myofiber differentiation. Here we show that Ozz targets the rod portion of embryonic myosin heavy chain and preferentially recognizes the sarcomeric rather than the soluble pool of myosin. We present evidence that Ozz binding to the embryonic myosin isoform within sarcomeric thick filaments marks it for ubiquitination and proteolytic degradation, allowing its replacement with neonatal or adult isoforms. This unique function positions Ozz within a system that facilitates sarcomeric myosin remodeling during muscle maturation and regeneration. Our findings identify Ozz-E3 as the ubiquitin ligase complex that interacts with and regulates myosin within its fully assembled cytoskeletal structure.
Highlights
Striated muscle cells exhibit the paradoxical association of a rigidly ordered fine structure with the ability to adapt their size and contractile properties during growth and development, or in response to changes in their patterns of use
We have shown earlier that the expression of ozz mRNA and Ozz protein increases during muscle development from embryonic day E12.5 onward [12] (Fig. 1A)
Ozz expression was downregulated during the phase of degeneration, but was upregulated during the regeneration phase (Fig. 1C), confirming that Ozz expression is associated with muscle development and growth
Summary
Striated muscle cells exhibit the paradoxical association of a rigidly ordered fine structure with the ability to adapt their size and contractile properties during growth and development, or in response to changes in their patterns of use. Mechanisms must exist to enable replacement of isoforms while maintaining an almost crystalline regularity of structural pattern. The ubiquitin-proteasome system [3,4,5] is the prime candidate for targeted degradation of most soluble and myofibrillar proteins. Evidence for ubiquitin-mediated degradation of myosin is mostly indirect, but the E3 ubiquitin ligases MuRF1, which is induced during muscle atrophy, and MuRF3 mediate the ubiquitination of soluble myosin in vitro [8,9], binding to multiple sites near the head region of MyHC molecules. Ubiquitination by MuRF1 has recently been shown to regulate the disassembly and degradation of the myofibrillar proteins MyBP-C, MLC1, and MLC2; MyHC is not ubiquitinated by MuRF1 in vitro when associated in the actomyosin complex or in the intact myofibrils [10]. Ubiquitin-dependent degradation has been indirectly implicated in the regulation of myosin folding and assembly [11]
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