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Relation of nebulin and connectin (titin) to dynamics of actin in nascent myofibrils of cultured skeletal muscle cells.

Cultured embryonic chicken skeletal muscle cells microinjected with rhodamine (rh)-labeled actin were stained with antibodies against nebulin and connectin (titin). In premyofibril areas, nebulin was observed as dotted structures, many of which were arranged in a linear fashion. These structures were associated with injected rh-actin. Among these linearly arranged dots of nebulin and rh-actin, numerous small nebulin dots without rh-actin incorporation were scattered. It is probable that the dots of nebulin and/or its associated protein(s) represent a preformed scaffold upon which actin monomers accumulate; exogenously introduced actin associates initially with small nebulin dots, which in turn coalesce to form rh-actin dots and are arranged linearly. In developing myofibrils, two patterns of nebulin distribution were found: "singlets" and "doublets." Recovery of rh-actin's fluorescence after photobleaching was slowest in the nonstriated dotted portions, followed by the striated myofibrillar portions with nebulin singlets and those with doublets, in that order. Thus, the distribution patterns of nebulin seem to be related to the accessibility/exchangeability of actin into nascent myofibrils. It is possible that early nebulin filaments exhibiting singlets are not tightly associated with actin filaments and that this loose association allows myofibrils to exchange nonadult isoforms of actin and other proteins into adult types. Connectin formed a striated pattern before the formation of rh-actin/nebulin striations. It appears that connectin does not have any significant role in the accessibility of actin into nascent myofibrils.