Regulatory mechanism for the function of myocardin family members and its relation to cell phenotype

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Myocardin (Mycd), which is essential for the differentiation of the smooth-muscle cell lineage, is constitutively located in the nucleus, although its family members, myocardin-related transcription factors A and B (MRTF-A/B), mostly reside in the cytoplasm and translocate to the nucleus in response to Rho-signaling. The mechanism for their nuclear import is unclear. Here, we investigated the mechanism for the nuclear import of Mycd family members, and demonstrated any correlation between such mechanism and the phenotype of vascular smooth muscle cells (VSMCs). In cultured VSMCs, the knockdown of importin β1 inhibited the nuclear import of Mycd and MRTF-A/B. Their NH2-terminal basic domain (NB) was identified as a binding site for importin α/β1 by in vitro analyses. However, Mycd had a higher affinity for importin α/β1 than MRTF-A/B did, even in the absence of G-actin, and Mycd's affinity for importin α1/β1 was stronger than for any other importin α/β1 heterodimers. The binding of Mycd to importin α/β1 was insensitive to G-actin, whereas that of MRTF-A/B was differently inhibited by G-actin. In dedifferentiated VSMCs, the levels of importins α1 and β1 were reduced concomitant with down-regulation of Mycd, serum response factor, and SMC markers. By contrast, in differentiated VSMCs, their expressions were up-regulated. Thus, the nuclear import of Mycd family members in VSMCs depends on importin α/β1, and their relative affinities for importin α/β1 heterodimers determine the Mycds' nuclear import. The expression of the Mycds' nuclear import machineries is related to the expression levels of VSMC phenotype-dependent SMC markers.