Fhos1 is a mammalian formin-family protein, and functions as an organizer of the actin microfilament. Here we have cloned human and mouse cDNAs for a novel Fhos homolog, designated Fhos2. The messages for Fhos2 are expressed in the heart, kidney, and brain, where the Fhos1 mRNAs are not abundant. Two splice variants of Fhos2 exist in a tissue-specific manner; the longer variant Fhos2L is the major form in the heart, whereas the kidney and brain predominantly express Fhos2S that encodes a shorter protein. Over-expression of an active form of the two Fhos2 variants, as well as that of Fhos1, induces the formation of actin stress fibers in HeLa cells, suggesting that Fhos2 acts as an actin-organizing protein. Biochemical analysis using rat cardiomyoblastic H9c2 (2-1) cells reveals that endogenous Fhos2 is enriched in the intermediate filament fraction. Consistent with this, Fhos2 localizes to the nestin intermediate filament but not to other cytoskeletons, as demonstrated by staining of H9c2 (2-1) cells with anti-Fhos2 antibodies. Furthermore, Fhos2 is present in nestin-expressing neuroepithelial cells of the fetal rat brain. Thus, Fhos2 not only has the actin-organizing activity but also associates with nestin, which may imply a Fhos2-mediated link between the nestin intermediate filament and actin microfilament.
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